OpenStep

OpenStep is an object-oriented application programming interface (API) specification developed by NeXT and Sun Microsystems, separating the application frameworks from the NeXTSTEP operating system so they could be implemented on multiple platforms. OpenStep is an object-oriented application programming interface (API) specification developed by NeXT and Sun Microsystems. It defines a framework for building graphical user interface (GUI) applications and was designed to be platform-independent, allowing software to run across multiple operating systems, including NeXTSTEP, Windows NT, and various Unix-based systems. OpenStep influenced later frameworks such as Cocoa for macOS and GNUstep.

With the introduction of the OpenStep specification, NeXT rebranded NeXTSTEP as OPENSTEP for Mach, its implementation of the specification for its Mach-based operating system, while maintaining compatibility with existing NeXTSTEP applications.