What is a major component that distinguishes a heat pump from an air conditioning system?

The major component that distinguishes a heat pump from an air conditioning system is the reversing valve.

Heat pumps have the unique ability to provide both heating and cooling by reversing the flow of refrigerant. The reversing valve is essential for this function; it changes the direction of the refrigerant flow so that the heat pump can either absorb heat from the outside air and transfer it inside (heating mode) or remove heat from indoor air and expel it outside (cooling mode). This versatility allows heat pumps to be used year-round, making them distinct from traditional air conditioning systems, which are designed only for cooling.

Other components such as the compressor, expansion valve, and accumulator play important roles in both systems but do not provide the same functionality that the reversing valve does in allowing for the dual operation of heating and cooling.