(Central Processing Unit) The computing part of the computer. Also called the "processor," the CPU is made up of the control unit, which executes the instructions, and the ALU, which performs the calculations and logical operations. Today, the CPUs of almost all computers are contained on a single microprocessor chip, and multiple processing units on one chip are commonplace (see multicore). See microprocessor, control unit and ALU.
Computer vs. Computer System
Technically, the CPU, clock and main memory make up a computer. In addition to the CPU, a complete computer system requires input/output channels, peripheral control units, storage devices and an operating system. In practice, the terms "computer" and "computer system" are synonymous.
From the Mainframer's Viewpoint
Computer professionals involved with mainframes refer to the entire computer as the CPU.