Humidity is the amount of water vapor in a gas such as a portion of the Atmosphere. The Temperature and makeup of the gas determines a maximum amount of water vapor the air can hold (Saturation), and a measure of humidity as a percentage of this amount is termed Relative Humidity. A measure based purely on mass, i.e., that of water vapor as a percentage of that of the total gas mixture, is termed Specific Humidity.
The concept of the humidity applies to the atmospheres of other worlds, thus are of interest in Atmospheric Models for them. This includes some solar system planets and Moons as well as some Extra Solar Planets. It may be used in analogy for other substances that can occur as either a gas or liquid on a world, e.g., for Methane on Titan.