Submodule
In plain terms
A repository nested inside another repository.
Going deeper
A Git submodule embeds one repository within another at a specific commit, letting you include external code while keeping histories separate. It links repos together.
Under the hood
Pins to a specific commit; powerful but notoriously tricky to keep in sync.
Examples
Shared libraries, vendored dependencies, nested repos, git submodule.
Think of it like...
Like citing a specific edition of another book inside your own.