Relative Humidity is the ratio of water vapor in the air in relation to the amount of water the air could hold at any given temperature and is expressed as a percent

For example, if a given volume of air at 75F can hold 4 grams of water vapor and there are only 3 grams of water in the air, then the Relative Humidity (RH) is 75%

The device used to measure humidity is called a hygrometer

The device used to regulate humidity is called a humidistat (or sometimes called a hygrostat)