Humidity is a term for the amount of water vapor
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...

 in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture, called the Absolute humidity. In everyday usage, it commonly refers to relative humidity, expressed as a percent in weather forecasts and on household humidistat
An electronic device analogous to a thermostat but which responds to relative humidity not temperature. Humidistats are utilized in a number of devices including dehumidifiers, humidifiers, and microwave ovens...

s; it is so called because it measures the current absolute humidity relative to the maximum.