1. A heat pump which transfers thermal energy from outside to inside a building, or vice versa.
2. A heat pump's refrigeration system consists of a compressor and two coils made of copper tubing (one indoors and one outside), which are surrounded by aluminum fins to aid heat transfer. In heating mode, liquid refrigerant in the outside coils extracts heat from the air and evaporates into a gas. The indoor coils release heat from the refrigerant as it condenses back into a liquid. A reversing valve, near the compressor, can change the direction of the refrigerant flow for cooling as well as for defrosting the outdoor coils in winter.
https://en.wikipedia.org/wiki/Air_source_heat_pumps (1.); https://energy.gov/energysaver/air-source-heat-pumps (2.)