API Library
In plain terms
Reusable code exposing functionality through an interface.
Going deeper
An API library packages functions and types developers call to use a capability without writing it themselves. It's prebuilt functionality you import.
Under the hood
Distinct from a framework (which calls your code); libraries are called by your code.
Examples
requests (Python), lodash, NumPy, HTTP clients, utility libraries.
Think of it like...
Like a toolbox of ready-made tools you pick up and use as needed.