Application Programming Interface (API) is a set of functions and procedures allowing the creation of applications that access the features or data of an operating system, application, or other service. When used as a question tag, please be sure to describe in your question which APIs are being used.

Application Programming Interface (API) is a set of functions and procedures allowing the creation of applications that access the features or data of an operating system, application, or other service.

When used on a question, this can refer to the various forms of SharePoint API (SSOM, SSOM, REST, etc.), a custom API being developed as part of a SharePoint solution, or an external API being consumed by a custom SharePoint solution.