Vapor

A vapor is a substance in a gaseous state at a temperature where a condensed phase can also be reached under suitable pressure conditions. For a pure substance below its liquid-gas critical point, isothermal compression can reach phase coexistence and then convert some vapor to liquid. At coexistence the equilibrium vapor pressure depends on temperature.

A sealed cell containing both water and water vapor illustrates the distinction between amount and state: at fixed temperature, changing the liquid fraction can leave the equilibrium vapor pressure unchanged while both phases remain. A dilute vapor can still approximate an ideal gas, so vapor does not mean that ideal-gas calculations are automatically invalid. Conversely, the broad term gas does not imply ideal behavior. Specifying composition, temperature and pressure makes the usage physically meaningful.