Glossary — H
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Hallucination (AI)
When an AI confidently states something that isn’t true. AI doesn’t know what it doesn’t know; it generates plausible-sounding text based on patterns from training. Hallucinations are most common with specific details: dates, statistics, citations, names, prices. The Mata v. Avianca case (2023) is the canonical example — a lawyer submitted AI-generated case citations that didn’t exist; he was sanctioned. Always verify specific factual claims from AI before relying on them.
See also: Hallucinations, Fact-check AI output
Hash
A function that takes any input (a password, a file, a JWT payload) and produces a fixed-size, seemingly random output. Same input → same output, always. Different input → totally different output, with overwhelmingly high probability. Used for passwords (with bcrypt/argon2), file integrity (SHA-256), and signatures (HMAC).
See also: Passwords & hashing
Header (HTTP)
Key-value metadata attached to an HTTP request or response. Examples: Content-Type: application/json, Authorization: Bearer <token>, Set-Cookie: .... Headers are how servers and clients communicate everything except the body of the message.
See also: HTTP
Hook (React)
A function in React that starts with use and lets a component “hook into” React features. useState is the most famous (component state). useEffect runs side effects after render. useMemo and useCallback optimize. Custom hooks (useMyThing()) compose existing hooks. Hooks have rules: only call them at the top level of a component, never inside conditionals or loops.
See also: React
Hook (Claude Code / Git)
In a different sense: a shell command that runs automatically in response to an event. Git has commit hooks (.git/hooks/pre-commit) that run before each commit. Claude Code has session hooks (in settings.json) that run on tool use or session events. Either way, a “hook” is a place to plug in extra behaviour.
See also: Hooks (Claude Code), Git basics
HTML
See the full textbook entry: HTML — HyperText Markup Language.
In one sentence: the markup language that describes the structure and content of every web page.
HTTP / HTTPS
HTTP (HyperText Transfer Protocol) is the protocol — the conversation format — used between browsers and web servers. A request has a method (GET, POST, etc.), a URL, headers, and an optional body. A response has a status code (200, 404, 500…), headers, and a body. HTTPS is HTTP wrapped in TLS encryption, so the conversation can’t be read or tampered with in transit. All modern websites use HTTPS.
See also: HTTP & HTTPS, HTTPS
Hydration
The process React/Next.js use to turn server-rendered HTML into a live, interactive React app in the browser. The server sends back HTML with markers; the browser downloads the JS, “attaches” event handlers, and the page becomes interactive. Hydration errors happen when the HTML the server rendered doesn’t exactly match what React expects on the client (often caused by using Date.now(), Math.random(), or browser-only APIs during render).
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