thumb|An ampule of [[nitrogen oxide vapor: brown nitrogen dioxide and colorless dinitrogen tetroxide, in equilibrium]]
In physics, a vapor (American English) or vapour (Commonwealth English; see spelling differences) is a substance in the gas phase at a temperature lower than its critical temperature, which means that the vapor can be condensed to a liquid by increasing the pressure on it without reducing the temperature of the vapor. A vapor is different from an aerosol.
For example, water has a critical temperature of , which is the highest temperature at which liquid water can exist at any pressure. In the atmosphere at ordinary temperatures gaseous water (known as water vapor) will condense into a liquid if its partial pressure is increased sufficiently.
A vapor may coexist with a liquid (or a solid). When this is true, the two phases will be in equilibrium, and the gas-partial pressure will be equal to the equilibrium vapor pressure of the liquid (or solid).
Examples
thumb|Invisible water vapor condenses to form visible water droplets called mist
- Perfumes contain chemicals that vaporize at different temperatures and at different rate in scent accords, known as notes.
- Atmospheric water vapor is found near the earth's surface, and may condense into small liquid droplets and form meteorological phenomena, such as fog, mist, and haar.
- Mercury-vapor lamps and sodium vapor lamps produce light from atoms in excited states.
- Flammable liquids do not burn when ignited. It is the vapor cloud above the liquid that will burn if the vapor's concentration is between the lower flammable limit (LFL) and upper flammable limit (UFL), of the flammable liquid.
E-cigarettes produce aerosols, not vapors.