Vapour

A vapour (British spelling) or vapor (see spelling differences) is a substance in the gas phase at a temperature lower than its critical point. This means that the vapour can be condensed to a liquid or to a solid by increasing its pressure without reducing the temperature.

For example, water has a critical temperature of 374 °C (647 K), which is the highest temperature at which liquid water can exist. In the atmosphere at ordinary temperatures, therefore, gaseous water (known as water vapor) will condense to liquid if its partial pressure is increased sufficiently.

A vapour may co-exist with a liquid (or solid). When this is true, the two phases will be in equilibrium, and the gas pressure will equal the equilibrium vapour pressure of the liquid (or solid).