vapor pressure

Force exerted by the gas or vapor released by a liquid or solid substance in a closed container or space. Vapor pressure is a relative measure of the tendency to evaporate (volatility), depends on the nature of the substance and ambient temperature, and reaches a state of equilibrium (where no more vapors escape from the substance) in closed spaces. Often expressed as pounds per square inch (PSI), it is stated usually in millimeters of mercury (mmHg) at 68°F (20°C) in a MSDS.