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In physical chemistry and in engineering, "steam" refers to vaporized water, typically at a temperature well above its sea level-pressure boiling point of 100 ° Celsius. It is a pure gas, which at atmospheric pressure occupies about sixteen hundred times the volume of liquid water. A steam engine uses the expansion of water into steam to drive a piston and so to perform work. In other industrial applications steam is used as a repository of energy, which is introduced and extracted by heat transfer, usually through pipes. Steam is a capacious reservoir for energy because of water's high latent heat of evaporation.

In common speach, "steam" most often refers to the white mist that condenses above boiling water as the hot vapor ("steam" in the first sense) mixes with the cooler air. After gaseous steam has intermixed with air, it is no longer properly called steam and is instead referred to as water vapor.

Also the nickname for Phil Shaw creator of the extreme sport extreme ironing.