What is the amount of water vapor in the air called?
This is known as the "humidity" of the air. Warmer air can hold more water than colder air.
The percentage of water vapor in the air, compared to the maximum amount possible at that temperature, is called the relative humidity.
Air that contains 100% of the possible water vapor is said to be saturated. In some cases, more water is added to the air, causing it to become supersaturated (relative humidity over 100%).