The Free On-line Dictionary of Computing (19 January 2023):

SunOS

    Sun Microsystems' version of Unix for
   Sun workstations.  SunOS is similar to BSD Unix with some
   SVR4 features and OpenWindows 3.0.

   After version 4, SunOS was integrated into Sun's Solaris
   "operating environment".

   (1999-03-26)