steam

thumb|Liquid phase eruption of Castle Geyser in [[Yellowstone Park]]

Steam is water vapor, often mixed with air and/or an aerosol of liquid water droplets. This may occur by evaporation or by boiling, where heat is applied until water reaches the enthalpy of vaporization. Superheated or saturated steam is invisible; however, wet steam, a visible mist or aerosol of water droplets, is often referred to as "steam".