Carnot theorem

No heat engine operating between two fixed reservoirs is more efficient than a reversible engine, and all reversible engines between those reservoirs have the same efficiency.

Absolute thermodynamic temperature can be defined so that a reversible engine exchanging heats $Q_h$ and $Q_c$ with reservoirs at $T_h$ and $T_c$ satisfiesA choice of one reference temperature fixes the scale.

A Carnot cycle consists of two reversible isothermal changes, one at each reservoir temperature, joined by two reversible adiabatic processes. It attains the maximum thermal efficiency permitted between those reservoirs, $1-T_c/T_h$.

An isothermal process is a thermodynamic change at constant temperature. In a reversible isothermal step, the working substance remains in thermal equilibrium with a reservoir while exchanging heat.