すいじょうき水蒸気suijouki
1.
The gaseous form of water that occurs when water evaporates, often invisible, but can condense into tiny droplets in the air, creating a mist-like appearance.
2.
A colorless and odorless gas produced from the transformation of liquid water through heat or boiling, which can sometimes be visible when it condenses and forms clouds or fog.
3.
An aerial form of water that results from evaporation, which may also refer to the fine droplets created when this gas cools and condenses in the atmosphere, commonly seen as steam or mist.
Example:
熱は水を水蒸気に変える。