This Claude Code glossary is a quick reference for the terms you keep running into when working with Anthropic's agentic coding tool — words like Agent Skills, hooks, MCP, subagents, and CLAUDE.md. Each entry is a short, plain-English definition so you can stop guessing what a feature does and get back to shipping. Where a topic deserves more depth, we link to a full deep dive and to the official docs. Claude Drops is an independent project and is not affiliated with Anthropic; for exact flags and syntax, always confirm against the official Claude Code docs.
Why this Claude Code glossary exists
Claude Code ships fast, and its vocabulary grows with it. This Claude Code glossary groups terms by theme: the core extensibility primitives first (Skills, slash commands, subagents, MCP, hooks), then the configuration and memory layer (CLAUDE.md, settings, permissions), then runtime concepts (plan mode, the context window, headless mode). If you just want a one-line lookup, jump to the quick-reference table at the end. For ongoing changes to any of these features, the Claude Code changelog is the source of truth, and you can get a push notification the moment a new release lands by grabbing the app.
Extensibility: Skills, commands, subagents, MCP, hooks
Agent Skills
Agent Skills are reusable, self-contained packages of instructions (and optionally scripts or resources) that teach Claude Code how to do a specific task well — for example, generating a changelog, scaffolding an API route, or following your team's release process. A skill typically lives in a folder with a markdown file describing when to use it and what steps to follow, and Claude loads it on demand when the task matches. Skills make repeatable workflows consistent across a team. For a full walkthrough, see Claude Code Skills explained.
Slash commands
Slash commands are shortcuts you type starting with / to trigger an action. Some are built in (for managing the session, configuration, and context), and you can define your own as reusable prompts. A custom slash command is a lightweight way to package a frequently used instruction so you don't retype it. Slash commands differ from Skills in scope: a slash command is usually a single prompt you invoke explicitly, while a Skill is a richer capability Claude can reach for automatically. See our slash commands guide for examples.
Subagents
Subagents are separate, focused Claude instances that the main agent can delegate work to, each with its own context window and often its own tools or instructions. Because a subagent works in an isolated context, it's useful for parallelizable or noisy tasks — searching a large codebase, running a review pass, or doing research — without flooding the main conversation. The parent agent receives the subagent's result, not its entire transcript. Learn more in Claude Code subagents explained.
MCP / MCP server
MCP stands for the Model Context Protocol, an open standard for connecting AI tools to external systems. An MCP server exposes a set of tools, resources, or prompts — for example a database, an issue tracker, a browser, or a design app — that Claude Code can call during a session. Adding an MCP server extends what Claude can actually do beyond reading and writing local files. Read the full primer in Claude Code MCP explained, and check the docs for how to configure servers.
Hooks
Hooks are shell commands that Claude Code runs automatically at defined points in its lifecycle — for instance before or after a tool runs, or when a session ends. Because the harness (not the model) executes them, hooks are how you enforce deterministic behavior: auto-format code after an edit, block writes to protected paths, run a linter, or log activity. Hooks turn "please remember to" into a guarantee. See Claude Code hooks explained for patterns and examples.
Configuration & memory
CLAUDE.md / memory
CLAUDE.md is a markdown file that Claude Code reads as persistent project memory — conventions, architecture notes, commands to run, and dos and don'ts. A project-level file lives in your repo and applies to that codebase; a user-level file can hold personal preferences that apply everywhere. Keeping CLAUDE.md tight and accurate is one of the highest-leverage things you can do, because its contents are loaded into context and steer every response.
Settings / settings file
Claude Code stores configuration — options such as permissions, hooks, environment variables, and model preferences — in a JSON settings file. Settings are typically layered across scopes: user-level settings that apply across projects, and project-level settings checked into the repo, so teams can share a baseline while individuals override locally. Because automated behaviors (hooks, allowed tools) are defined here, the settings file is effectively the control panel for the harness. See the docs for the current file locations and keys.
Permissions / allowlist
The permission system governs which actions Claude Code can take without asking — running specific commands, editing files, or calling certain tools. An allowlist (allowed tools) is the set of operations pre-approved so they run without a prompt, which is what makes longer autonomous runs practical. Conversely, you can deny or require confirmation for sensitive operations. Tuning permissions is a balance between fewer interruptions and keeping a human in the loop for risky actions.
Runtime concepts
Plan mode
Plan mode is a way to have Claude Code investigate and propose an approach before it changes anything. The agent reads code and explains what it intends to do, and you approve the plan before edits or commands run. It's ideal for larger or higher-stakes changes where you want to review the strategy first rather than discover surprises mid-edit.
Output styles
Output styles let you adjust how Claude Code communicates — for example, more concise responses, or a different tone or format for explanations — without changing what it's able to do. They're a presentation-layer preference, useful for matching your workflow or for teaching contexts where you want more explanation. Check the docs for the styles currently available and how to select one.
Context window & compaction
The context window is the amount of text — your messages, files, tool results, and Claude's replies — the model can consider at once. Long sessions can fill it up, so Claude Code uses compaction: summarizing earlier parts of the conversation to free room while preserving the important details. This explains why very long sessions sometimes "forget" minor details, and why a clear CLAUDE.md (which is reloaded) is more durable than something said hundreds of messages ago.
Headless mode
Headless mode runs Claude Code non-interactively — typically from a script, CI pipeline, or another program — so it executes a task and returns a result without a live chat session. This is the foundation for automation: scheduled jobs, code review bots, batch refactors, or any workflow where no human is sitting at the prompt. See the docs for the exact invocation and output options.
The changelog
The changelog is the running list of what changed in each Claude Code release — new features, fixes, and behavior changes. Anthropic publishes it as CHANGELOG.md on GitHub. Because the tool ships frequently, keeping an eye on the changelog is the best way to learn about new primitives early. Claude Drops mirrors it in a browsable format at the Claude Code changelog and can notify you when a new version drops.
Quick-reference table
A one-line lookup for the most common terms. Use the sections above for fuller explanations and links to deep dives.
| Term | One-line definition |
|---|---|
| Agent Skill | Reusable package of instructions that teaches Claude a specific task. |
| Slash command | A typed /shortcut that triggers a built-in or custom action. |
| Subagent | A separate Claude instance with its own context for delegated work. |
| MCP server | An external service exposing tools/resources Claude can call. |
| Hook | A shell command the harness runs automatically at lifecycle points. |
| CLAUDE.md | Markdown project/user memory loaded into context every time. |
| Settings file | JSON config for permissions, hooks, env vars, and preferences. |
| Permissions / allowlist | Controls which actions run without asking. |
| Plan mode | Propose an approach for approval before making changes. |
| Output styles | Adjust how Claude communicates, not what it can do. |
| Context window | How much text the model can consider at once. |
| Compaction | Summarizing older conversation to free context space. |
| Headless mode | Run Claude Code non-interactively from scripts or CI. |
| Changelog | The running list of changes in each release. |
Where to go next
This glossary is a map, not the territory — each term has more depth than a paragraph can hold. The fastest way to keep your mental model current is to skim new releases as they ship. Browse the Claude Code changelog to see what's new, dive into a deep dive on Skills, MCP, or hooks, or grab the app to get a push notification the moment a new Claude Code version lands. For exact syntax, the official docs are always the final word.
Maintainer, Claude Drops
Ian builds Claude Drops and reads every Claude Code release so you don't have to. He writes plain-English guides to Claude Code's features, drawing directly from the official changelog and documentation.