Thermal efficiency and the coefficient of performance for devices is a measure of their performance.

They are defined on the basis of the first law only, and they are sometimes referred to as the first-law efficiencies.

The first law efficiency, however, makes no reference to the best possible performance, and thus it may be misleading.

Second-law efficiency is defined as the ratio of the actual thermal efficiency to the maximum possible (reversible) thermal efficiency under the same conditions.

$$η_Π = \frac{η_{th}}{η_{threv}} ........ (For Heat Engine)$$