The mass of water vapor per unit volume of air is known as humidity. However, there are different types of humidity used to measure the amount of water vapor present in the air.

1. Relative humidity: This is the most commonly used measure of humidity. It is the ratio of the amount of water vapor present in the air to the maximum amount of water vapor the air can hold at a specific temperature. Relative humidity is expressed as a percentage and indicates how close the air is to being saturated with water vapor.

2. Specific humidity: This is the actual mass of water vapor present in the air per unit mass of air. It is expressed in grams of water vapor per kilogram of air and does not change with temperature. Specific humidity is often used in meteorology and is useful for determining the amount of moisture present in the atmosphere.

3. Absolute humidity: This is the mass of water vapor present in a unit volume of air. It is expressed in grams of water vapor per cubic meter of air. Absolute humidity gives a direct measure of the actual amount of water vapor in the air.

4. Variable humidity: This is not a recognized term for measuring humidity. Alert - correct answer should be Specific humidity