OpenStep

OpenStep, also stylized as OPENSTEP, is an open source API derived from the NeXTSTEP operating system that was developed by NeXT.

After Apple Computer announced in December 1996 that it would acquire NeXT, OpenStep was used to develop Mac OS X, codenamed Rhapsody, as Apple's next operating system. iOS and macOS (formerly Mac OS X) can trace their developmental roots to NeXTSTEP as their APIs are prefixed with NS (NSObject, NSString, etc.).