The instruments that measure water vapor content in the air are called a hygrometer and psychrometer. Water vapor is water suspended in the atmosphere in the form of an invisible gas. Humidity is the measure of water vapor content of the air.
Humidity is usually expressed as a ratio of the amount of water vapor present in a given volume or parcel of air compared to the total amount of water vapor that same unit of air could actually hold. Hygrometers that are designed to be used in homes are often mounted together with two other weather instruments. It is often mounted with a thermometer to measure room air temperature and a barometer to measure air pressure.