Humidity is the amount of water vapor in the air. Water vapor is the gaseous state of water and is invisible.
There are three main measurements of humidity: absolute, relative and specific. Absolute humidity is the water content of air at a given temperature expressed in gram per cubic meter. Relative humidity, expressed as a percent, measures the current absolute humidity relative to the maximum (highest point) for that temperature. Specific humidity is a ratio of the water vapor content of the mix to the total water content on a small basis.