Microsoft's application programming interfaces available in the Windows operating system.

The Windows API is Microsoft's set of application programming interfaces. With them, you can do cool stuff like adding and removing user accounts, shutting down the system, and creating and managing windows on the screen.

From Wikipedia:

The Windows API, informally WinAPI, is Microsoft's core set of application programming interfaces (APIs) available in the Microsoft Windows operating systems. The name Windows API collectively refers to a number of different platform implementations that are often referred to by their own names (for example, Win32 API). Almost all Windows programs interact with the Windows API; on the Windows NT line of operating systems, a small number (such as programs started early in the Windows startup process) use the Native API.