Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the ratio of the partial pressure of water vapor in the air-water mixture to the saturated vapor pressure of a flat sheet of pure water at those conditions. The relative humidity of air depends not only on temperature but also on the pressure of the system of interest. Relative humidity is often used instead of absolute humidity in situations where the rate of water evaporation is important, as it takes into account the variation in saturated vapor pressure.

Note that the definition of relative humidity does not depend on the presence of other gases (such as air) along the water vapor—only two partial pressures of water vapor are involved in it. According to a common misconception, air is said to be unable to 'hold' any more water vapor than 100% relative humidity. In practice, however, relative humidity can exceed 100%. In nature, it does so in virtually all clouds, where it is often around 101%. In a laboratory, it is not dificult to fill a container with pure water vapor at 300% relative humidity.