Glossary
Plain-English definitions of every concept the offlineutils.com tools touch — what each one is, how it works, when to reach for it, and which tool on this site operates on it.
All glossary terms
CIDR
aka Classless Inter-Domain Routing, CIDR notation, slash notation, prefix length
CIDR (Classless Inter-Domain Routing) is the notation that writes a block of IP addresses as an address followed by a slash and the number of fixed leading bits — 192.168.1.0/24 — replacing the rigid class A/B/C system with prefixes of any length.
Cron Expression
aka Crontab, Cron schedule, Cron pattern
A cron expression is a five-field string (or six fields with the Quartz seconds extension) that describes a recurring schedule — used by Unix cron, Kubernetes CronJobs, GitHub Actions schedules, and most modern job runners to say things like 'every 15 minutes' or 'every weekday at 9am'.
Cryptographic Hash
aka Hash function, Digest, Message digest, Checksum
A cryptographic hash function takes any input and produces a fixed-length pseudo-random output (the 'digest') that uniquely identifies the input — and is computationally infeasible to reverse, making hashes the foundation of integrity checks, content addressing, and password storage.
EXIF
aka Exchangeable Image File Format, EXIF metadata
EXIF (Exchangeable Image File Format) is the metadata standard cameras and phones embed inside JPEG photos — including GPS coordinates, camera model, capture timestamp, and shutter settings — which is why a holiday photo can quietly leak the exact street address it was taken at.
Floating-point number
aka IEEE 754, double precision, binary64, float64, Number.MAX_SAFE_INTEGER, safe integer
A floating-point number stores a value as a binary fraction scaled by a power of two. JavaScript's Number is an IEEE 754 double with 53 significant bits, so integers above 2^53 − 1 (9,007,199,254,740,991) and most decimal fractions, 0.1 included, cannot be stored exactly.
Homograph attack
aka IDN homograph attack, homoglyph attack, script spoofing, lookalike domain, confusable domain
A homograph attack uses characters that look identical to others — Cyrillic а in place of Latin a — so that a different domain, username or identifier appears to be a trusted one. Browsers defend domain names by showing the xn-- form of names that look suspicious instead of the Unicode spelling.
JSON
aka JavaScript Object Notation, RFC 8259 JSON
JSON (JavaScript Object Notation) is a lightweight text format for structured data — built from objects, arrays, strings, numbers, booleans, and null — that's become the lingua franca of web APIs, configuration files, and data interchange between services.
JSON Web Token
aka JWT, JOSE JWT, JSON Web Token (RFC 7519)
A JSON Web Token (JWT) is a compact, URL-safe credential made of three Base64URL-encoded segments — header, payload, and signature — that an API issues so a client can prove who it is on subsequent requests without re-authenticating each time.
OKLCH
aka OKLCH color space, oklch() CSS color function
OKLCH is a perceptually uniform color space — built on the OKLab perceptual model — that lets designers reason about colors using lightness, chroma, and hue values where equal numeric changes correspond to equal perceived changes. Modern CSS exposes it via the oklch() color function.
aka Portable Document Format, PDF file, ISO 32000
PDF (Portable Document Format) is a page-description file format that locks the exact layout, fonts, and graphics of a document so it renders identically on any device — which is why contracts, invoices, bank statements, and forms are almost always shared as PDFs.
ULID
aka Universally Unique Lexicographically Sortable Identifier
A ULID (Universally Unique Lexicographically Sortable Identifier) is a 128-bit identifier encoded as 26 Crockford base32 characters that is sorted by time when generated, making it a drop-in replacement for UUID v4 with much better database insert performance.
Unicode normalization
aka Unicode normalisation, NFC, NFD, NFKC, NFKD, UAX #15, Unicode equivalence
Unicode normalization rewrites text into one standard sequence of code points, so strings that look identical — é stored as a single precomposed character, or as e followed by a combining accent — compare as equal. Unicode defines four normalization forms: NFC, NFD, NFKC and NFKD.
Unix Timestamp
aka Unix time, epoch time, POSIX time, epoch seconds, time_t
A Unix timestamp is a single number counting the seconds elapsed since 1970-01-01T00:00:00Z, ignoring leap seconds — a time zone-free, language-agnostic way to name an exact instant, which is why it is what logs, databases, and APIs store internally.
UUID
aka Universally Unique Identifier, GUID, RFC 4122 UUID
A UUID (Universally Unique Identifier) is a 128-bit value, usually written as 32 hex digits in the canonical 8-4-4-4-12 grouping (e.g. '550e8400-e29b-41d4-a716-446655440000'), used as a globally unique ID that any computer can generate without coordination.