The Carnot cycle is a theoretical thermodynamic cycle that consists of four reversible processes, which are:

Isothermal Expansion: The working substance (e.g. a gas) is placed in contact with a heat reservoir at a high temperature, and heat is transferred to the working substance, causing it to expand while maintaining a constant temperature. This is a reversible process because the heat is transferred slowly enough to maintain thermal equilibrium throughout the expansion.

Adiabatic Expansion: The working substance is now thermally isolated (i.e., no heat can enter or leave the system), but it continues to expand, causing its temperature to decrease. This process is reversible because no heat is exchanged, and the expansion is slow enough to maintain thermal equilibrium.

Isothermal Compression: The working substance is now placed in contact with a heat reservoir at a low temperature, and heat is transferred out of the system, causing it to compress while maintaining a constant temperature. This is a reversible process because the heat is transferred slowly enough to maintain thermal equilibrium throughout the compression.

Adiabatic Compression: The working substance is now thermally isolated, but it continues to be compressed, causing its temperature to increase. This process is reversible because no heat is exchanged, and the compression is slow enough to maintain thermal equilibrium.

The Carnot cycle is a highly efficient cycle that represents the maximum efficiency that any heat engine operating between two temperatures can achieve, given that it operates in a reversible manner.