A heat pump is a machine or device that effectively "moves" thermal energy

Thermal energy

Thermal energy is the part of the total internal energy of a thermodynamic system or sample of matter that results in the system's temperature....

 from one location called the "source," which is at a lower temperature, to another location called the "sink" or "heat sink

Heat sink

A heat sink is a term for a component or assembly that transfers heat generated within a solid material to a fluid medium, such as air or a liquid. Examples of heat sinks are the heat exchangers used in refrigeration and air conditioning systems and the radiator in a car...

", which is at a higher temperature. An air conditioner is a particular type of heat pump, but the class includes many other types of devices. During the operation, some of the thermal energy must be transformed to another type of energy in the process, before reappearing as thermal energy in the sink.

The heat pump uses mechanical work

Mechanical work

In physics, work is a scalar quantity that can be described as the product of a force times the distance through which it acts, and it is called the work of the force. Only the component of a force in the direction of the movement of its point of application does work...

, or some source of thermodynamic work (such as much higher-temperature heat source dissipating heat to lower temperatures) to accomplish the desired transfer of thermal energy from source to sink.