The Carnot cycle is a theoretical thermodynamic cycle that demonstrates the maximum possible efficiency of a heat engine. It consists of four reversible processes: two isothermal (constant temperature) and two adiabatic (no heat exchange). The cycle operates between two heat reservoirs, absorbing heat from a hot reservoir and releasing it to a cold reservoir. The efficiency of the Carnot cycle depends on the temperatures of the two reservoirs. It is defined as the ratio of work output to heat input, and it shows that no engine can be more efficient than a Carnot engine operating between the same temperatures. This principle helps in understanding the limits of real-world engines.