Humidity is the concentration of water vapor in the air. The concentration can be expressed as absolute humidity, specific humidity, or relative humidity. The amount of humidity in an air controlled environment is an important issue to the HVAC industry. Humidity can make the temperature of the surrounding air feel like it is warmer than the actual temperature.
A common expression in some countries is It's not the heat, it's the humidity, in reference to unpleasantly muggy weather. This expression refers to the fact that humid air can be significantly less comfortable than drier air at the same temperature.