[OkayPhysicist]

APIs are simply a property of any system you want a program to interact with. The confusion comes from the synecdoche of referring to web applications that are only interacted with via their API as APIs.

The more interesting one is frameworks and libraries. There's a gradient between them based, IMO, on how opinionated they are. Working with a framework means filling in the blanks, e.g. "When we recieve an http request to this endpoint, this application code should run" in contrast to a library which is just a barrel of functions to call. But that's a fuzzy line.