A coprocessor is a computer processor used to supplement the functions of the primary processor (the CPU). For example, the Intel 80386 microprocessor used an optional "math" coprocessor (the 80387) to perform floating point operations directly in hardware.
Sometimes an entire computer is made out of coprocessors instead of a single CPU. This is the case in the Nintendo 64 video game console, for example. Because of today's highly advanced degree of miniaturization in VLSI circuits, it is relatively rare to see coprocessors in computer architecture anymore.