/ GLOSSARY

Glossary of technical terms

Plain-English definitions for 80 terms that come up across the tools on this site: image formats, PDF standards, data encodings, cryptography, color, web development, time, finance, health, math, and barcodes. Every entry links back to the tool where the concept is put to work.

Last reviewed May 25, 2026.

A

Amortization: The process of paying down a loan over time with regular installments. Each payment covers a mix of interest and principal. Early payments are mostly interest because the outstanding balance is still large; later payments retire more principal as the balance shrinks. An amortization schedule lists every payment in order with running totals. Mortgages, car loans, and most personal loans follow this pattern, while credit cards and revolving lines of credit do not. See the mortgage calculator for a worked schedule.
Annual percentage rate (APR): The yearly cost of borrowing money, expressed as a percentage of the loan amount and including most fees rolled in by the lender. APR is a closer-to-true price than a quoted interest rate alone, because it folds in origination fees, points, and required insurance. Two loans with the same nominal interest rate can have very different APRs once fees enter the calculation. Disclosure rules in many countries require lenders to publish APR alongside the headline rate. Try the loan calculator.
ASCII: A 7-bit character encoding from 1963 that assigns numbers 0 to 127 to English letters, digits, punctuation, and control codes such as newline and tab. ASCII has no accents, no emoji, and no scripts beyond basic Latin. Every modern text encoding, including UTF-8 and the Windows code pages, keeps ASCII as its first 128 values, which is why plain English text from any era still opens correctly. The HTML entities toolshows how non-ASCII characters are escaped for the web.
Aspect ratio: The proportional relationship between the width and height of an image, video, or screen, written as W:H. Common ratios are 16:9 for widescreen video, 4:3 for older displays, 1:1 for social media squares, and 3:2 for most still cameras. Cropping changes the aspect ratio; resizing preserves it. Picking the wrong ratio for a target platform causes letterboxing or unwanted cropping. The image crop tool includes presets for the most common ratios.
AVIF: AV1 Image File Format. A modern image format derived from the AV1 video codec, offering smaller files than JPEG and WebP at the same perceived quality. It supports HDR, wide color gamuts, alpha transparency, and animation. AVIF has good browser support since 2022 but decoding is slower than JPEG, so it is best used where bandwidth matters more than CPU. Use the image converter to convert into and out of AVIF.

B

Basal metabolic rate (BMR): The number of calories your body burns at complete rest to keep basic functions running: breathing, blood circulation, cell repair, and temperature regulation. BMR is the floor of daily energy expenditure before any activity is added. It is usually estimated from height, weight, age, and sex using formulas such as Mifflin-St Jeor or Harris-Benedict. Multiplying BMR by an activity factor gives total daily calorie need. See the calorie calculator.
Base64 encoding: A way of representing binary data as ASCII text using 64 printable characters: A-Z, a-z, 0-9, plus, and slash. Each three input bytes become four output characters, growing the payload by roughly 33 percent. Base64 is used for embedding images directly in CSS or HTML, for sending binary data inside JSON, and inside email attachments. It is an encoding, not encryption: anyone can decode it. The base64 encoder and decoderconverts in both directions.
Body fat percentage: The share of your total body mass that is fat tissue, as opposed to lean mass (muscle, bone, water, and organs). A more informative number than weight alone, since two people of the same weight can have very different body compositions. Direct measurement requires DEXA scanning or hydrostatic weighing; estimates can be made from skinfold calipers or US Navy tape measurements at the neck, waist, and hips. Calculate yours with the body fat calculator.
Body Mass Index (BMI): A weight-for-height ratio used as a quick population-level screen for underweight, normal weight, overweight, and obesity. Calculated as weight in kilograms divided by height in metres squared. BMI does not distinguish fat from muscle, so very athletic people often score "overweight" despite low body fat. It is best read as one data point among several, alongside waist circumference and body fat percentage. The BMI calculator shows the standard category bands.

C

Chmod / Unix permissions: The classic Unix system that grants read, write, and execute rights to the file owner, the owner's group, and everyone else. Often expressed as a three-digit octal number such as 755 (executable directory) or 644 (readable file). Each digit adds 4 for read, 2 for write, and 1 for execute. The chmod command applies a new permission set. Setting permissions too loose exposes files; too tight, the program cannot run. The chmod calculator converts between octal and rwx.
CIDR notation: Classless Inter-Domain Routing. A compact way of writing an IP network as an address followed by a slash and a prefix length: 192.168.1.0/24 means the first 24 bits identify the network and the remaining 8 bits identify hosts within it. CIDR replaced the older class-based system of A, B, and C networks. Smaller prefix numbers cover more addresses: a /16 holds 65,536; a /30 holds just four. Use the CIDR calculator to expand a block.
Color gamut: The full range of colors that a display, printer, or color space can reproduce. Larger gamuts include more saturated colors. The common gamuts are sRGB (web standard, smallest), Display P3 (modern phones and laptops), Adobe RGB (print preflight), and Rec.2020 (HDR video). A wide-gamut display can also misrepresent narrower content if color management is sloppy. When colors look wrong on different screens, mismatched gamuts are often the cause. The color converter works in sRGB hex.
Complementary colors: Two colors directly opposite each other on the color wheel, such as red and cyan, blue and orange, or yellow and purple. Placing them together produces maximum visual contrast; mixing them in equal parts in additive color produces white, and in subtractive color produces grey. Designers use complementary pairs sparingly to highlight a focal point without overwhelming the eye. Build a palette around them with the color palette generator.
Compound interest: Interest calculated on the original principal plus any previously accumulated interest, as opposed to simple interest which only ever applies to the original principal. Compounding causes balances to grow exponentially rather than linearly over time, which is powerful for investments and punishing for debts. The frequency of compounding (daily, monthly, yearly) matters: more frequent compounding produces a higher effective yield from the same nominal rate. See the compound interest calculator.
Content Security Policy: An HTTP response header that tells a browser which sources of scripts, styles, images, and outbound connections to allow on a page. A strict CSP blocks injected scripts before they execute, which mitigates cross-site scripting and unauthorised data exfiltration. CSP also supports frame-ancestors to control embedding and report-to to send violation reports for monitoring. inyourbrowser.com uses a strict CSP; see the verify-local page for a live demonstration.
Cron expression: A five- or six-field string that schedules a recurring task on a Unix-like system. Fields run minute, hour, day-of-month, month, and day-of-week, each accepting numbers, ranges, lists, and the * wildcard. An expression like 0 9 * * 1-5 means every weekday at 09:00. Many job schedulers, CI systems, and serverless platforms accept cron syntax. The order and meaning of fields varies slightly between implementations. Parse one with the cron expression parser.
Cross-origin request: A web request from one origin (scheme plus host plus port) to a different one. Browsers restrict cross-origin reads by default to protect users from rogue scripts on one site reading data from another. The target server must opt in via CORS (Cross-Origin Resource Sharing) headers such as Access-Control-Allow-Origin for the read to succeed. Strict CSP further limits which origins a page can even attempt to reach. See the verify-local page for examples.
CSV: Comma-Separated Values. A plain-text table format where each line is a row and commas separate fields. Values containing commas, quotes, or newlines must be wrapped in double quotes, with inner quotes doubled. CSV is simple and widely supported by spreadsheets and databases, but the spec (RFC 4180) is informal and tools disagree on edge cases like leading zeros and date formats. Convert CSV to and from JSON with the CSV to JSON converter.

D

Data URI: A URL scheme that embeds the resource itself directly in the link, using the data: prefix followed by a MIME type and base64-encoded payload. A small PNG can be written as data:image/png;base64,iVBORw0K... and used anywhere a regular URL would be. Data URIs let a single HTML file include images and fonts inline, which is handy for offline pages, but they bloat caches because each copy is unique. The image to base64 tool generates data URIs.
Daylight saving time: The practice of shifting clocks forward one hour in spring and back one hour in autumn to extend evening daylight. DST start and end dates vary by country and have changed over time, which is why software relies on the IANA timezone database to map a local time to UTC correctly for any historical date. The European Union, the United States, and most of Australia observe DST; most of Asia and Africa do not. Convert across zones with the timezone converter.
Diff algorithm: An algorithm that finds the minimal set of insertions and deletions to transform one sequence (often lines of text) into another. The classic Myers algorithm runs in linear time on average and underpins git diff and most text diff tools. Output is typically rendered side-by-side or inline with added and removed lines highlighted. Different diff modes operate on lines, words, or characters depending on how fine-grained the comparison should be. Compare two texts with the text diff.

E

EAN-13 barcode: European Article Number, 13 digits. The barcode used on retail products worldwide outside North America. The first two or three digits are a GS1 country prefix (not necessarily the country of manufacture), the next group identifies the manufacturer, then the product, and the final digit is a check digit computed from the first twelve. EAN-13 is a superset of the older UPC-A. Generate one with the barcode generator.
Embedded font: A font whose glyphs are stored inside a document (typically a PDF) rather than referenced by name and hoped to exist on the reader's machine. Embedding guarantees that the file looks the same wherever it is opened, at the cost of a larger file size. Fonts can be fully embedded or subset-embedded, the latter shipping only the glyphs the document actually uses. Licensing restrictions on some fonts forbid embedding. PDF/A and PDF/X require all fonts to be embedded.
Epoch time: A way of expressing a moment in time as the number of seconds (or milliseconds) since a fixed reference point called the epoch. The Unix epoch is 1970-01-01 00:00:00 UTC, the most common reference; other systems use the start of 1601 (Windows FILETIME), 1900 (NTP), or 2001 (Apple Cocoa). Epoch time is timezone-free and easy for computers to compare and store. Convert between epoch and human-readable form with the Unix timestamp converter.
EXIF data: Exchangeable Image File Format. A metadata standard embedded inside JPEG, TIFF, and HEIC files. EXIF records the camera model and lens, exposure settings, the moment the shutter opened, and often GPS coordinates if the device has location services on. EXIF is invaluable for sorting and editing photos, but it can leak personal information when photos are shared publicly. Many social platforms strip EXIF on upload; many tools that process images preserve it unless told otherwise.

G

Greatest common divisor (GCD): The largest positive integer that divides two or more numbers without remainder. The GCD of 12 and 18 is 6. The classic way to compute it is the Euclidean algorithm, which repeatedly replaces the larger number with the remainder of dividing the larger by the smaller until the remainder is zero. GCD is used to reduce fractions to lowest terms, check coprimality, and in cryptographic algorithms such as RSA. See the fraction calculator.

H

Hash collision: When two different inputs produce the same hash output. Cryptographic hashes are designed so that finding a collision is computationally infeasible: an attacker would need to try an impractical number of inputs. MD5 collisions were demonstrated in 2004 and SHA-1 in 2017, which is why neither is suitable for signatures or password storage today. SHA-256 and SHA-512 remain collision-resistant. Compute hashes with the hash generator.
HEIC: High Efficiency Image Container, the iPhone default format since iOS 11. HEIC wraps HEVC-encoded images and offers roughly half the file size of JPEG at comparable visual quality. It supports HDR, wide color, live photos with depth maps, and image sequences. Browser and non-Apple platform support is partial because HEVC is patent-encumbered, which is why many people convert HEIC to JPEG or PNG before sharing. The image converter handles HEIC input.
HEX color: A way of writing a sRGB color as a # followed by three or six hexadecimal digits encoding red, green, and blue from 00 to FF: pure red is#FF0000. An eight-digit form adds an alpha (opacity) channel. Three-digit shorthand like #F00 expands by doubling each digit. HEX is concise and unambiguous, which is why it dominates web design and CSS. Convert to RGB, HSL, or named colors with the color converter.
HSL color model: Hue, saturation, lightness. A cylindrical color model where hue is an angle from 0 to 360 degrees around the color wheel (0 red, 120 green, 240 blue), saturation is a percentage of greyness, and lightness runs from 0 (black) through 50 (pure color) to 100 (white). HSL maps better to how people think about color than RGB does, making it easier to pick a lighter or darker variant. CSS supports hsl() natively. See the color converter.
HSV color model: Hue, saturation, value, also called HSB for brightness. A cylindrical color model similar to HSL but with a different lightness axis: value runs from 0 (black) to 100 (the fully bright form of the chosen hue), instead of HSL's 0 (black) to 100 (white). HSV is the model exposed by most color picker dialogs because it matches how mixing pigment feels: choose a hue, then dial down saturation toward grey or value toward black. The color converter handles HSV.
HTML entity: A short escape sequence that represents a character with special meaning in HTML. Reserved characters like &, <, and > must be written as &, <, and > so the parser does not treat them as markup. Named entities cover the most common characters; any Unicode code point can be written numerically as &#N; or&#xH;. Encode and decode with the HTML entities tool.
HTTP header: A key-value pair sent at the start of an HTTP request or response that carries metadata about the message: content type, length, encoding, caching rules, cookies, authentication tokens, and security policies. Headers come in standard and custom flavours; custom ones are conventionally prefixed with X- though that practice is now discouraged by RFC 6648. Response headers like Content-Security-Policy and Strict-Transport-Security carry critical security guarantees.

I

IANA timezone database: Also called tzdata or the Olson database. The community-maintained database of every named timezone the world has used, including historical changes to offsets and daylight saving rules. Names look like Europe/London or America/Los_Angeles. Operating systems and language standard libraries ship a copy that is updated several times a year as governments change DST rules. Use it whenever you need accurate local times in the past. Try the timezone converter.
ISO 8601: The international standard for date and time strings, designed to be sortable as text and unambiguous across cultures. It specifies the YYYY-MM-DD date format and theYYYY-MM-DDTHH:MM:SSZ combined timestamp format, with the trailingZ meaning UTC. Time zone offsets like +02:00 can replace Z. Most APIs, log formats, and modern programming languages default to ISO 8601 for serialised timestamps. See the Unix timestamp tool for conversions.

J

JPEG: Joint Photographic Experts Group. A lossy raster image format optimised for photographs, using discrete cosine transform compression that throws away high-frequency detail the human eye is least sensitive to. Quality is tunable from blocky to near-original. JPEG has been the dominant photograph format on the web for over thirty years because it is universally supported and produces small files for real-world photos. Compress JPEG with the image compressor.
JSON: JavaScript Object Notation. A lightweight text format for structured data: strings, numbers, booleans, null, arrays, and objects with string keys. JSON descends from JavaScript's object literal syntax but is a separate, language-independent standard (RFC 8259) supported by every modern programming environment. It has largely replaced XML for web APIs because it is compact, easy to read, and a near-perfect match for typical program data structures. Format JSON with the JSON formatter.
JWT: JSON Web Token. A compact, signed token used for authentication and authorization in web APIs. A JWT has three base64url-encoded parts separated by dots: a header naming the signing algorithm, a payload of claims (subject, issuer, expiry), and a signature over the first two. The signature lets a server verify a token came from a trusted issuer without a database lookup. JWT payloads are not encrypted; anyone holding the token can read the claims. Inspect one with the JWT decoder.

L

Least common multiple (LCM): The smallest positive integer divisible by each of two or more given numbers. The LCM of 4 and 6 is 12. LCM and GCD are related: for any two positive integers a and b, the product of a and b equals their GCD multiplied by their LCM. LCM is used when adding fractions with different denominators (find the common denominator) and when working out when two cyclical events next coincide. See the fraction calculator.
Levenshtein distance: The minimum number of single-character insertions, deletions, or substitutions needed to transform one string into another. The distance from "kitten" to "sitting" is 3. Levenshtein distance is the classic metric behind fuzzy matching, spell-checkers, and DNA sequence alignment. The straightforward dynamic- programming implementation runs in O(m*n) time. Related variants include Damerau- Levenshtein (which counts transpositions as one edit) and Jaro-Winkler.
Lossless compression: A compression technique that reduces file size without discarding any data, so the decompressed file is bit-for-bit identical to the original. Used by PNG, FLAC, ZIP, and most general-purpose archive formats. Lossy formats like JPEG and MP3 throw away some data to get smaller files. For photographs lossy is usually fine; for line art, screenshots, source code, and text, lossless preserves sharpness and reproducibility. Try the image compressor.
Lossy compression: Compression that achieves smaller files by permanently discarding information humans are least likely to notice: high-frequency image detail, very quiet audio. JPEG, MP3, AAC, WebP at low quality, and most video codecs are lossy. The trade-off is dramatic: a lossy JPEG can be one-tenth the size of a lossless PNG of the same picture with no obvious difference. Repeated saves compound the loss, so always keep an original. Compress images with the image compressor.

M

Magic bytes: A short fixed byte sequence at the start of a file that identifies its format, independent of the filename extension. PDF starts with %PDF, PNG with the bytes 89 50 4E 47 0D 0A 1A 0A, ZIP with PK. Used because filename extensions can lie or be missing, while magic bytes travel with the data. The Unix file command, browser MIME sniffing, and most upload validators consult magic bytes to decide what a file really is.
MD5: Message Digest 5. A 128-bit cryptographic hash function designed by Ron Rivest in 1992. MD5 has been broken since 2004 for collision resistance: researchers can produce two different inputs that hash to the same digest in seconds on modern hardware. It is therefore unsuitable for signatures, password hashing, or anything security-related, but it is still widely used as a fast integrity checksum to detect accidental file corruption. Compute one with the hash generator.

N

Net present value: The current value of a stream of future cash flows, discounted back to today using a chosen interest rate (the discount rate). A positive NPV means an investment is expected to produce more value than its cost in today's money; a negative NPV means the reverse. NPV is the cornerstone of corporate finance project evaluation. The choice of discount rate is critical and usually reflects the cost of capital or the return on a risk-free alternative. Estimate returns with the ROI calculator.

O

One-rep max: The maximum weight a lifter can move for a single repetition of a given exercise. The gold standard for assessing absolute strength, used to prescribe percentages for training programmes. Direct testing is risky and tiring, so 1RM is usually estimated from a heavier-weight, multi-rep set using formulas such as Epley, Brzycki, or Lombardi. The estimates diverge above 10 reps and should be re-tested as strength changes. Calculate yours with the one-rep max calculator.
Optical character recognition (OCR): Software that converts an image of text (a scan, photograph, or screenshot) into machine-readable text. Modern OCR uses convolutional and recurrent neural networks and reaches near-human accuracy on clean printed text in major languages. Accuracy degrades with low resolution, skewed pages, ornate fonts, and handwriting. OCR is what turns a scanned PDF from a flat image into a document you can search and copy from. Extract already-embedded text with the PDF text extractor.

P

PDF form: A PDF document with interactive fields, such as text boxes, checkboxes, radio buttons, and signature lines, that a user can fill in. Two flavours exist: AcroForm, the original PDF feature defined in the ISO PDF spec, and XFA, an Adobe XML-based extension that ISO PDF 2.0 deprecates and most non-Adobe readers no longer support. Filled values can be saved in the PDF or exported as FDF or XML. AcroForms can also be flattened so the values become part of the page.
PDF page object: An internal PDF structure representing a single page. Stores the page size and orientation (the MediaBox), rotation, links to its content stream of drawing instructions, and references to the fonts, images, and other resources used on that page. Pages are children of the Pages tree at the root of the document. Operations like rotate, reorder, or delete pages work on these objects directly. Try the PDF reorder tool.
PDF watermark: Text or an image drawn over or under each page of a PDF to mark ownership, status (DRAFT, CONFIDENTIAL), or origin. A watermark can be a true overlay (added as a separate annotation or optional content group, easy to remove) or stamped permanently into the page content stream so it survives flattening and printing. Decorative watermarks deter casual copying; they are not a security mechanism on their own. Add one with the PDF watermark tool.
PDF/A: An ISO standard subset of PDF (ISO 19005) designed for long-term archiving. PDF/A forbids reliance on external resources, requires every font to be embedded, bans JavaScript, encryption, and audio or video. The goal is that the file should render identically a century from now on hardware nobody has yet imagined. Several conformance levels exist: PDF/A-1 (most restrictive), through PDF/A-3 (allows attached source files). Many national archives mandate PDF/A for deposited records.
PDF/X: An ISO standard subset of PDF (ISO 15930) designed for print production exchange. PDF/X mandates embedded fonts, requires a specified output intent (the target press condition, such as ISO Coated v2), and disallows screen-only features like transparency in older versions. Several variants exist: PDF/X-1a for CMYK and spot color, PDF/X-4 for files with live transparency. Most commercial printers accept or require PDF/X-formatted files to avoid surprises on press.
Pixel density: The number of pixels packed into a unit of physical space on a display, usually measured in pixels per inch (PPI) or dots per inch (DPI). A standard desktop monitor sits around 90-110 PPI; modern phones reach 400-500 PPI; Apple's "Retina" displays began around 218 PPI. Higher density renders text and images more crisply but multiplies the asset sizes needed: a 2x display wants images at double the logical pixel dimensions. See image resizer.
PNG: Portable Network Graphics. A lossless raster image format with full alpha transparency, designed in the mid 1990s to replace the patent-encumbered GIF. PNG compresses sharp edges, flat color regions, and text well, which makes it the standard format for screenshots, line art, logos, and icons on the web. Files are larger than JPEG for photographs, where the lossless guarantee earns its keep less. Compress PNGs with the image compressor.

Q

QR code error correction: Redundancy built into a QR code using Reed-Solomon codes so that the code can still be decoded when partially damaged, dirty, or covered by a logo. Four levels are defined: L (7% recoverable), M (15%), Q (25%), and H (30%). Higher levels add more redundancy at the cost of a denser pattern for the same payload. Use L for printed marketing where damage is unlikely; use H for stickers in rough environments or designs with a logo overlay. See the QR code generator.

R

Raster vs vector: Two fundamentally different ways of describing images. Raster formats (JPEG, PNG, BMP, WebP) store a fixed grid of pixels; enlarging them produces visible blockiness or blurring. Vector formats (SVG, EPS, PDF paths) store shapes as mathematical paths and fills that can scale to any size without quality loss. Photographs are inherently raster; logos, icons, and diagrams belong in vector. Both can coexist inside a single PDF or SVG document.
Regular expression: A pattern language for matching text. Combines literal characters with metacharacters such as . (any character), * (zero or more), + (one or more), [abc] (character class), ^ (start), and $(end) to describe complex string patterns concisely. Variants include POSIX, PCRE (used by most modern languages), and JavaScript's ECMAScript flavour, each with small but real differences. Test patterns with the regex tester.
Return on investment (ROI): A profitability ratio comparing the net gain from an investment to its cost. Expressed as a percentage: (gain - cost) / cost * 100. Simple ROI ignores time, which can mislead when comparing investments held for very different periods, so annualised ROI is often more useful. ROI also ignores risk, currency effects, and opportunity cost. It is a quick first pass, not a complete decision metric. Use the ROI calculator.
RGB color model: Additive color model where every color is a mixture of red, green, and blue light. Each channel runs from 0 to 255 in 8-bit form; 10-bit and 16-bit variants exist for HDR and professional imaging. RGB is the model of displays, cameras, and scanners. Subtractive models like CMYK are used for ink on paper. RGB without a defined color space is ambiguous, which is why color management tags an image with its space (sRGB, Display P3, Adobe RGB). See the color converter.
Roman numeral: The numeral system of ancient Rome, using the letters I (1), V (5), X (10), L (50), C (100), D (500), and M (1000) to write numbers. Combinations follow additive and subtractive rules: VII is 5 + 1 + 1 = 7, while IV is 5 - 1 = 4. The system has no zero and becomes unwieldy above a few thousand. Still used for clock faces, book volume numbers, monarch names, and movie copyright years. Convert with the Roman numeral converter.

S

Salt (cryptography): Random data added to a password before hashing it, so that two users with the identical password produce different hashes. The salt is stored alongside the hash; it is not secret. Salts defeat precomputed rainbow tables and force an attacker who steals the password database to attack each password individually rather than all at once. Modern password hashes (bcrypt, scrypt, argon2) handle salting automatically and add deliberate slowness on top.
SHA-256: Secure Hash Algorithm 256. A cryptographic hash function from the SHA-2 family that produces a 256-bit (32-byte) output. SHA-256 is the current standard for general-purpose digital signatures, blockchain proof-of-work, certificate signing, and file integrity checking. No practical collisions are known; the security margin is expected to last for decades. The output is usually displayed as 64 hexadecimal characters. Compute one with the hash generator.
SHA-512: A cryptographic hash function from the SHA-2 family that produces a 512-bit (64-byte) output. Internally it operates on 64-bit words, which can be faster than SHA-256 on modern 64-bit CPUs despite the larger digest. SHA-512 offers a higher security margin than SHA-256 and is widely used in OpenPGP and some operating system password hashing. SHA-384 and SHA-512/224 are truncated variants for cases where a smaller digest is wanted. Compute with the hash generator.
SQL formatter: A program that reformats SQL queries for readability: consistent keyword case (typically uppercase), aligned columns in SELECT lists, indented sub-queries and CTEs, and one clause per line. Formatters help with code review, version control diffs, and debugging long statements written by ORMs or query builders. Most dialects share enough grammar that a single formatter handles MySQL, PostgreSQL, SQL Server, and SQLite with small option toggles. Try the SQL formatter.
sRGB: Standard Red Green Blue. The standard color space for the web and most consumer displays, defined in 1996 by HP and Microsoft and adopted as IEC 61966-2-1. sRGB covers a smaller gamut than Display P3, Adobe RGB, or Rec.2020, but it is the assumption when an image carries no color profile and is therefore the safe default for content meant to look consistent everywhere. CSS color values are sRGB unless an explicit color space is given. Use the color converter.
Subnet mask: A 32-bit number that separates the network and host portions of an IPv4 address. Written as four dotted octets like 255.255.255.0 or as a CIDR prefix like /24. Bits that are 1 mark the network portion; bits that are 0 mark the host portion. To find the network address a router does a bitwise AND of the IP and the mask. Modern routing uses CIDR notation, but the dotted form still appears in many home router UIs. Try the CIDR calculator.
Subtractive notation: The Roman numeral rule of placing a smaller value before a larger one to subtract it: IV for 4, IX for 9, XL for 40, XC for 90, CD for 400, CM for 900. Used to avoid long runs of the same letter, since IIII is harder to read than IV. Strict rules limit subtractive pairs to I before V or X, X before L or C, and C before D or M. Inscriptions and clock faces sometimes ignore the rules (a classic clock writes 4 as IIII). The Roman numeral converter handles both forms.

T

TSV: Tab-Separated Values. Like CSV but with tab characters as the field separator. TSV is less ambiguous than CSV because data rarely contains tabs naturally, so quoting is often unnecessary. Spreadsheets export and import TSV when copying and pasting between Excel and the clipboard, which is why pasting from Excel into a code editor produces tab- delimited rows. Many bioinformatics file formats are TSV under the hood. Convert with the CSV to JSON converter, which handles TSV too.

U

Unicode: An international standard that assigns a unique number (code point) to every character in every writing system, plus emoji and symbols. Unicode currently covers over 150,000 characters across 168 scripts, ancient and modern. Code points are written as U+0041for "A". The standard is maintained by the Unicode Consortium and grows several times a year. UTF-8, UTF-16, and UTF-32 are different ways of encoding the same code points as bytes.
Unix timestamp: A count of seconds (sometimes milliseconds in JavaScript) elapsed since the Unix epoch, 1970-01-01 00:00:00 UTC. The universal way of storing a moment in time on computers, because it is timezone-free, monotonic, and easy to compare. 32-bit signed Unix timestamps overflow at 03:14:08 UTC on 19 January 2038, the "Year 2038 problem"; 64-bit timestamps push the overflow billions of years away. Convert with the Unix timestamp converter.
UPC-A barcode: Universal Product Code, version A. The 12-digit barcode used on retail goods in the United States and Canada since 1974. A UPC-A is mathematically a subset of EAN-13 with an implicit leading zero, which is why a scanner that reads EAN-13 also reads UPC-A. The first six digits identify the manufacturer (assigned by GS1), the next five the product, and the last is a check digit. Generate one with the barcode generator.
URL encoding: Also called percent encoding. The way characters with special meaning in URLs (or non- ASCII characters generally) are escaped using a percent sign followed by two hex digits: space becomes %20, the ampersand & becomes %26. Defined by RFC 3986. Required for any data that flows through a URL path, query string, or form-encoded request body. Encode and decode with the URL encoder.
UTF-8: Unicode Transformation Format, 8-bit. A variable-width encoding of Unicode that uses 1 to 4 bytes per character. ASCII-compatible: the first 128 code points encode identically as single bytes, so plain English text in UTF-8 is byte-identical to ASCII. UTF-8 is the default encoding of the modern web (over 98% of pages) and of every major operating system. It is self-synchronising, meaning a corrupted byte affects at most one character.

V

vCard format: A standard text format for storing contact information: name, phone numbers, email addresses, postal addresses, photos, and more. Files have the .vcf extension and start with BEGIN:VCARD. vCard is defined by RFC 6350 (version 4) and is the format behind most contact import and export across phones, mail clients, and address books. Encoding a vCard in a QR code lets a phone import a business card with a single scan. See the QR code generator.

W

WCAG contrast ratio: A ratio measuring the relative luminance difference between two colors, computed by the formula in WCAG 2.1. Values run from 1:1 (no contrast, same color) to 21:1 (pure white on pure black). Body text needs at least 4.5:1 to meet WCAG Level AA and 7:1 to meet AAA; large text gets a lower threshold of 3:1 and 4.5:1 respectively. Sufficient contrast is essential for users with low vision and helpful for everyone in bright sunlight. Check pairs with the contrast checker.
Web Crypto API: A browser API that exposes cryptographic primitives to JavaScript: hashing (SHA-256, SHA-384, SHA-512), HMAC, AES symmetric encryption, RSA and elliptic-curve signatures and key exchange, and cryptographically secure random number generation. The API is backed by audited native implementations, which is far safer and faster than reimplementing crypto in JavaScript. inyourbrowser.com uses Web Crypto for the hash generator and the password generator.
WebP: An image format from Google, first released in 2010, derived from the VP8 video codec. WebP supports both lossy and lossless compression, alpha transparency, and animation. Files are typically 25-35% smaller than JPEG or PNG at the same perceived quality, which is why it has become a default on the web. Browser support is universal since 2020. WebP sits between the older JPEG and PNG and the newer AVIF in compression efficiency and adoption. Convert with the image converter.
Word boundary: In regular expressions, the zero-width position between a word character (letter, digit, or underscore) and a non-word character (anything else, including the start or end of the string). Written as \b. Useful for matching whole words: \bcat\bmatches "cat" but not "catalog" or "scatter". The exact definition of "word character" varies between regex flavours; Unicode-aware engines extend it to letters from any script. Test with the regex tester.

X

XML: Extensible Markup Language. A verbose text format with nested tags, attributes, and a strict schema mechanism (XSD, DTD, or RELAX NG). XML was dominant for data interchange in the 2000s and remains the basis of major formats such as SVG, RSS, SOAP, Office Open XML (.docx, .xlsx), and many configuration files. JSON has displaced XML for new web APIs because it is shorter and maps directly to JavaScript objects. Format and validate with the XML formatter.

Y

YAML: YAML Ain't Markup Language. A human-friendly data format using indentation instead of brackets, with optional inline JSON-like syntax for short values. Common in configuration files for Kubernetes, GitHub Actions, Docker Compose, and most modern CI systems. The reliance on whitespace makes YAML easy to read but easy to break: a single wrong indent can change the meaning of a document. The YAML 1.2 spec is fully a superset of JSON. Format with the YAML formatter.