A device that moves heat from one location to another. An air conditioning system is a form of heat pump. Heat from inside the aircraft cabin is absorbed by a liquid refrigerant. When the refrigerant absorbs the heat, it evaporates, or changes into a gas. The gas is then compressed, and the compression raises its temperature higher than that of the outside air.

The compressor forces the hot gas through the condenser, located outside the aircraft. Here, the gas transfers some of its heat to the outside air and condenses back into a liquid. Heat in this way is pumped from inside the aircraft cabin to the air outside the aircraft.