(Originally) water droplets suspended in air, especially in the form of mist or steam (now rare); (in later use) water in its (invisible) gaseous phase.
Origin
Late 15th century.
Definition of water vapour in US English:
water vapour
(USwater vapor)
noun
(Originally) water droplets suspended in air, especially in the form of mist or steam (now rare); (in later use) water in its (invisible) gaseous phase.