Central Processing Unit
The part of a computer which controls all the other parts. Designs vary widely but, in general, the CPU consists of the control unit, the arithmetic and logic unit (ALU) and memory (registers, cache, RAM and ROM) as well as various temporary buffers and other logic.
The control unit fetches instructions from memory and decodes them to produce signals which control the other part of the computer. This may cause it to transfer data between memory and ALU or to activate peripherals to perform input or output.
A parallel computer has several CPUs which may share other resources such as memory and peripherals.
The term "processor" has to some extent replaced "CPU", though RAM and ROM are not normally considered as part of a processor. This is particularly true of common modern microprocessors though there have been microprocessors which include RAM and/or ROM on the same integrated circuit.