Dependency
In plain terms
External code a project relies on to function.
Going deeper
A dependency is a library or package your code needs to work, managed and installed by package managers. Your project leans on it.
Under the hood
Transitive dependencies cascade; managing versions and vulnerabilities is a key supply-chain concern.
Examples
npm packages, pip requirements, Maven dependencies, libraries, lockfiles.
Think of it like...
Like ingredients a recipe requires that you must source before cooking.