An API is an application programming interface. This document defines several key terms related to APIs: 1. A protocol is a set of rules governing data exchange between devices, like HTTP. 2. An object can be a material thing or a data construct used by a computer, like those defined in libraries. 3. REST is an architectural style that follows specific construction methods, used in this class. The document then discusses libraries and frameworks, distinguishing that frameworks prescribe certain usage while libraries only provide reusable code. It identifies jQuery and Bootstrap as examples. Finally, it outlines different types of APIs like built-in browser, web service, library, and third-party APIs.