Source : Free On-Line Dictionary of Computing

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)