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

coprocessor

   Any computer processor which assists the main processor (the
   "CPU") by performing certain special functions, usually much
   faster than the main processor could perform them in software.
   The coprocessor often decodes instructions in parallel with
   the main processor and executes only those instructions
   intended for it.

   The most common example is a floating point coprocessor (or
   "FPU"), others are graphics and networking.

   (1995-01-05)