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.
Try this search on Wikipedia, OneLook, Google
Nearby terms: copious free time « copper « Copper Distributed Data Interface « coprocessor » copy and paste » copybook » copybroke