In thermodynamics, efficiency, symbol η (eta), is the ratio of work output to heat input of any heat engine: [1]

$\eta = \frac{W_{out}}{Q_{in}}$

The maximum theoretical efficiency that any heat engine can obtain is based on Carnot’s theorem.

References
1. Perrot, Pierre. (1998). A to Z of Thermodynamics. Oxford University Press.

External links
● Thermal efficiency – Wikipedia.