The Free On-line Dictionary of Computing (30 December 2018):

OpenStep

    An object-oriented application
   programming interface (API) derived from NEXTSTEP and
   proposed as an open standard by NeXT in 1994.

   OpenStep is the specification of the object kits of NEXTSTEP.
   OPENSTEP/Mach was an implementation of this specification.
   The original, OPENSTEP version 4.0, and really was NEXTSTEP 4.
   Rhapsody was the codename for Apple's Mac OS X Server,
   which is really NEXTSTEP 5 (it calls itself "kernel 5.3" at
   boot time).

   OpenStep was designed to be implemented independently of the
   computer's operating system, hardware, and user interface.
   The API for Rhapsody will be a superset of OpenStep's.

   When the OpenStep API is implemented for a specific platform
   and made into a product, it is written in uppercase,
   e.g. OPENSTEP Developer 4.2 for Mach, or OPENSTEP Enterprise
   for Windows NT and Windows 95.  Versions of OPENSTEP exist
   for Windows 95/NT, Solaris, HP/UX, and Mach.

   (1999-11-25)