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, code named Rhapsody, as Apple's next operating system. iOS and macOS can trace their developmental roots to NeXTSTEP as their APIs are prefixed with NS (NSObject, NSString, etc.).