AppKit

AppKit (formally Application Kit)[1] is a graphical user interface toolkit.

Later, AppKit and Foundation became part of Cocoa, the Objective-C API framework of macOS.

AppKit comprises a collection of Objective-C classes and protocols that can be used to build an application in OpenStep/Cocoa.

A sign of the NeXTSTEP heritage, AppKit's classes and protocols still use the "NS" prefix.

Most of the applications bundled with macOS—for example, the Finder, TextEdit, Calendar, and Preview—use AppKit to provide their user interface.