Note however that there may also be instances where a polytropic process occurs in a non-ideal gas.

A polytropic process is a thermodynamic process that obeys the relation:

Note that for specific polytropic indexes a polytropic process will be equivalent to a constant-property process.

A polytropic process, in particular, causes changes to the system so that the quantity is constant (where is pressure, is volume, and is the polytropic index, a constant).

The most important polytropic processes run between the adiabatic and the isotherm functions, the polytropic index is between 1 and the adiabatic exponent (γ or κ)