Vapor or vapour refers to the gaseous state of matter.

Although both words are frequently used interchangeably, vapor often carries the connotation of gaseous matter in a state of equilibrium with identical matter in a liquid or solid state. See the entry on vapor pressure for more information on this topic.

The constituent atoms or molecules of a vapor possess vibrational, rotational, and translational motion. More information can be found under the entry of the Kinetic theory of gases.