Abbreviated API. The complete set of all operating system functions that an application can use to perform such tasks as managing files and displaying information.
An API provides a standard way to write an application, and it also describes how the application should use the functions it provides. Using an API is quicker and easier than developing functions from scratch and helps to ensure some level of consistency among all the applications developed for a specific operating system.
In operating systems that support a graphical user interface, the API also defines functions to support windows, icons, drop-down menus, and other components of the interface. In network operating systems, an API defines a standard method that applications can use to take advantage of all the network features.