The ratio of the partial pressure of water vapour in an air-water mixture to the saturated vapour pressure of water at a given temperature is termed as -

Relative humidity, the ratio of the actual vapor pressure of water in the air to that in air saturated with water vapor; it is often expressed as a percentage.
Absolute humidity, the vapor concentration or density in the air.
Specific humidity, the mass of water vapor in a unit mass of moist air, usually expressed as grams of vapor per kilogram of air, or, in air conditioning, as grains per pound.
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