1. The portion of a computer that includes circuits controlling the interpretation and execution of instructions. 2. The portion of a computer that executes programmed instructions, performs arithmetic and logical operations on data, and controls input/output functions. Synonym central processor. 3. Abbreviation for communications processor unit. A computer embedded in a communications system, i.e., the portion of a digital communications switch that executes programmed instructions, performs arithmetic and logical operations on signals, and controls input/output functions.