Reference

Glossary

One word per idea. Every lesson uses these terms and avoids the aliases in italics.

The core model

Commit: A saved snapshot of the whole project, identified by a hash (e.g. a1f3c2e), that points back to its parent commit(s). Avoid: save, version.
Branch: A lightweight, movable pointer to one commit; when you commit on a branch, the pointer moves forward to the new commit (Pro Git 3.1). Avoid: copy, fork (a fork is a whole copy of a repository).
HEAD: Git's pointer to the branch you are currently on — the branch your next commit will move.
Default branch (main): The branch GitHub shows visitors, clones check out, and pull requests target by default; GitHub names it main (GitHub Docs). Avoid: master (older Git default), trunk.
Feature branch: A short-lived branch holding one self-contained change, merged into main when done. Avoid: topic branch, dev branch.
Working tree: The files you see and edit on disk; switching branches rewrites the working tree to match the branch's commit. Avoid: workspace, local copy.

Merge: Bringing the commits of one branch into the current branch.
Fast-forward merge: A merge where the current branch has not moved since the other branch started, so Git simply slides the pointer forward — no new commit.
Merge commit: A commit with two parents that joins two diverged branches.
Merge conflict: A merge Git cannot finish automatically because both branches changed the same lines differently; Git marks the spot with <<<<<<<, =======, >>>>>>>.
Squash merge: Merging a pull request by combining all its commits into one new commit on the base branch.
Three-dot diff: git diff main...feature: only the changes made on feature since it split from main — what a pull request shows.

Merge base: The most recent commit that two branches share: the point where one split from the other. git merge-base main feature prints it (git merge-base). Avoid: fork point (Git uses that name for something else).
Two-dot diff: git diff main..feature: how the tip of main and the tip of feature differ right now, the same as git diff main feature. Work that landed on main after the split shows up reversed.
Stale branch: A branch whose merge base with main is no longer the tip of main, because main moved after the branch split or was last updated. Its review and test results describe an older main. Avoid: outdated, old branch. GitHub says "behind" or "not up to date with the base branch".
Baseline: The score you subtract from a new score to see what changed. To see what a branch did, take the baseline at the merge base. Avoid: reference score, control.

Remote (origin): Another copy of the repository, usually on GitHub; origin is just the default name git clone gives it.
Remote-tracking branch: A read-only local pointer such as origin/main recording where a branch was on the remote the last time you talked to it.
Upstream: The remote branch a local branch is linked to, so plain git push and git pull know where to go; set with git push -u. Avoid: tracking target.
Pull request (PR): A GitHub proposal to merge a head branch into a base branch, with a diff, discussion, and checks.
Head branch / base branch: In a pull request, the head branch holds the proposed changes; the base branch (usually main) receives them.
Draft pull request: A pull request marked work-in-progress; it cannot be merged until marked ready for review.

Mainline: The single shared branch (main) that represents the current state of the product (Fowler).
Continuous integration (CI): Merging small changes into the mainline frequently, each one verified by an automated build and tests.
Continuous deployment (CD): Automatically building, testing, and publishing every change that lands on the mainline.
Workflow: A GitHub Actions automation defined in .github/workflows/*.yml, triggered by events such as push or pull_request. Avoid: pipeline (fine elsewhere, but not GitHub's term).
Status check: A pass/fail result (often a workflow job) reported on a commit and shown on the pull request.
Branch protection: Repository rules (protection rules or rulesets) that block changes to a branch unless conditions are met — e.g. a pull request with passing status checks.

Agent branch: In this course: a feature branch created for one Claude Code task, so the agent's commits are isolated and reviewable as a pull request.
Worktree: An extra working tree attached to the same repository, with its own branch checked out, so two sessions can work side by side (git worktree).
Checkpoint: Claude Code's automatic, session-level snapshot of files before each prompt, restorable with /rewind; not a replacement for commits (Claude Code docs).

Name trap: Claude Code's /branch command branches the conversation, not git (Claude Code commands).