define efficiency of carnot engine?

Carnot's theorem is a formal statement of this fact: No engine operating between two heat reservoirs can be more efficient than a Carnot engine operating between the same reservoirs.

This maximum efficiency is defined to be:

where

W is the work done by the system (energy exiting the system as work),
Q_H is the heat put into the system (heat energy entering the system),
T_C is the absolute temperature of the cold reservoir, and
T_H is the absolute temperature of the hot reservoir.