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. 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.