Vapor, the gaseous form of substances that are usually liquids or solids. The process by which a substance becomes a vapor is called vaporization. The changing of a liquid into a vapor is also called evaporation. The reverse process—that of changing a vapor into a liquid—is called condensation. The term sublimation is used to refer both to the process by which a solid changes directly into a vapor and to the process by which a vapor changes directly into a solid.

Water vapor and the evaporation of water greatly influence weather and climate. Vaporization has important applications in distillation and refrigeration.