BTU stands for British Thermal Unit. It is a unit of measurement for heat energy, specifically defined as the amount of heat required to raise or lower the temperature of one pound of water by one degree Fahrenheit

. BTUs are commonly used to quantify the energy used or transferred by heating and cooling systems such as air conditioners, furnaces, and heat pumps. For example, in air conditioning, the BTU rating indicates how much heat the unit can remove from indoor air in one hour, reflecting its cooling capacity

. In heating, it measures the heat output capacity of furnaces or other heating devices

. In summary, BTU is a standard measure of thermal energy used primarily in HVAC and energy-related applications to describe heating or cooling power. One BTU is roughly equivalent to 1,055 joules in the metric system