The term 'THE 'U' OF CPU OR BTU' is an acronym for the word 'UNIT'. UNIT is a term used to describe a measure of a physical quantity, such as length, mass, time, temperature, electric current, and luminous intensity. In computing, the UNIT of CPU or BTU stands for Central Processing UNIT or British Thermal UNIT, respectively. The Central Processing UNIT is the main component of a computer system that carries out instructions and processes data. The British Thermal UNIT is a UNIT of energy used to measure the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit.