What is the name of the parameter that measures the departure of a real gas from ideal gas behavior?

The parameter that measures a real gas’s departure from ideal-gas behavior is the compressibility factor.

Usually written as Z, the compressibility factor is defined by Z = pV/(nRT), or equivalently as the ratio of a gas’s actual molar volume to the ideal molar volume at the same temperature and pressure. An ideal gas has Z = 1 by definition. Values above or below 1 indicate non-ideal behavior.

At relatively high pressure, molecular repulsions often make the real gas occupy more volume than an ideal gas, producing Z greater than 1. At lower pressures and moderate temperatures, attractive intermolecular forces can dominate, giving Z less than 1. Deviations become especially important near a substance’s critical point or boiling point.

The term is not the same as compressibility itself. Compressibility is a response to pressure change, while the compressibility factor is a dimensionless correction to the ideal gas law. Engineers obtain Z from equations of state, experimental data, or generalized compressibility charts, with the value depending on temperature, pressure, and gas composition.