WHAT IS A HEAT PUMP?
A heat pump is an electrically-powered refrigeration cycle device which is capable of providing both heating and cooling to a residence. In the summer, it operates like an air conditioner by "moving" heat from inside the house to the outside. In the winter, the refrigerant flow is reversed, "moving" heat into the house by further chilling the already cold outside air and transferring the heat gained in the process to the inside.
Turner, Larry W., "Energy Fact Sheet: Heat Pumps for Residential Heating and Cooling: Some Questions and Answers" (1979). Agricultural Engineering Energy Series. 26.