Steam

is vaporized water. It is a transparent gas. At standard temperature and pressure, pure steam (unmixed with air, but in equilibrium with liquid water) occupies about 1,600 times the volume of an equal mass of liquid water. In the atmosphere, the partial pressure of water is much lower than 1 atm, therefore gaseous water can exist at temperatures much lower than 100 °C (212 °F) (see

Steam phase eruption of Castle Geyser in Yellowstone Park

water vapor and humidity).

In common speech, steam most often refers to the visible white mist that condenses above boiling water as the hot vapor mixes with the cooler air. This mist consists of tiny droplets of liquid water. Pure steam emerges at the base of the spout of a steaming kettle where there is no visible vapor.