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

RISC System/6000
RS6000
RS/6000
RS6K

    (Or "RS/6000") IBM's current RISC-based Unix
   computer.  The RS/6000, announced in 1990, replaced the
   RT-PC.  It runs AIX 3.x and 4.x.  Most models have an
   MCA bus.  A wide range of models are available.

   (1995-04-06)