[metiscus]

In physics we call a device that uses the compression and expansion of a fluid to move heat between parts of a system a heat pump.

An air conditioner uses a compressor to convert a refrigerant and sends the compressed fluid through a condenser. The condenser rejects heat from the system into the environment. This is the component of an air conditioner that is found outside. The compressed fluid is then passed through an expansion valve and into an evaporator where it is allowed to expand. Expansion is a process that requires heat. The heat flows into the expanding fluid from the environment inside the home. Air is blown across the evaporator coils to transfer heat energy from the home into the fluid which is then returned outside.

A heat pump is capable of reversing the flow of heat energy. The flow of fluid is reversed from an air-conditioner using something called a reversing valve. The compressor sends compressed fluid into the home where heat is rejected through the evaporator coil. The fluid then flows into the condenser coil and is allowed to expand outside, drawing in heat from the outside environment. The heated fluid is then returned to the compressor and the cycle continues.

In HVAC terminology an Air Conditioner is a one way physical heat pump and a heat pump is a bidirectional physical heat pump. Hopefully that helps clear it up a bit.