API

API is an acronym for Application Programming Interface. It is a set of clearly defined methods of communication between the service and any other software or components. An API defines the data structures of the program.

A Web API is a source code interface that a computer system uses to support requests for services to be made by a computer program. Web APIs deliver your request to the service provider, and then deliver the response back to you.

For instance, Twitter's REST API allows developers to access core Twitter data. And the Search API provides methods for developers to interact with Twitter Search and trends data.