The term PART OF BTU is composed of two crossword solutions: THERMAL (7 letters) and UNIT (4 letters). BTU stands for British THERMAL UNIT, which 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. The first solution, THERMAL, refers to the fact that BTUs are used to measure THERMAL energy. The second solution, UNIT, refers to the fact that BTUs are a UNIT of measurement.