A thermal reservoir is an idealized body large enough to absorb or supply heat without appreciably changing its temperature.

A heat engine operates cyclically, receives heat $Q_H$ from a high-temperature reservoir, rejects heat $Q_L$ to a lower-temperature reservoir, and produces net work

$$W_{net}=Q_H-Q_L.$$

Its thermal efficiency is

$$\eta_{th}=\frac{W_{net}}{Q_H}=1-\frac{Q_L}{Q_H}.$$

A refrigerator uses work to remove heat from a cold region. Its coefficient of performance is

$$COP_R=\frac{Q_L}{W_{in}}.$$

A heat pump is the same cyclic device judged by the heating delivered to the warm region:

$$COP_{HP}=\frac{Q_H}{W_{in}}=COP_R+1.$$

Unlike an efficiency, a COP can exceed one because it compares heat moved with work supplied; the device is not creating energy.

These performance definitions apply to any cyclic technology. Specific cycles determine the state changes that produce the values of $Q$ and $W$.