Steam is the technical term for the gaseous phase of water, which is formed when water boils. Technically speaking, in terms of the chemistry and physics, steam is invisible and cannot be seen; however, in common language it is often used to refer to the visible mist or aerosol of water droplets formed as this water vapor condenses in the presence of (cooler) air. At lower pressures, such as in the upper atmosphere or at the top of high mountains water boils at a lower temperature than the nominal 100 °C (212 °F) at standard temperature and pressure. If heated further it becomes superheated steam.
The enthalpy of vaporization is the energy required to turn water into the gaseous form when it increases in volume by 1,700 times at standard temperature and pressure; this change in volume can be converted into mechanical work by steam engines such as reciprocating piston type engines and steam turbines, which are a sub-group of steam engines.