Vapour is a mass of very small drops of liquid hanging in the air. Most often it is there because the liquid was heated — the steam over a pot of soup, the white cloud above a bathtub, the breath you can see on a cold January morning in Taipei. In science, vapour also means the gas form of something that is normally liquid or solid: water vapour is water in the air, invisible when it is well mixed, visible when it gathers into drops.

This is the British spelling. Americans write vapor. Both are completely correct; just don't switch between them inside one essay.

A point students find surprising: the white cloud you see over a kettle is not really the vapour. True water vapour is invisible. What you see is the vapour cooling and turning back into tiny drops. So strictly, the visible white is condensed vapour — but nobody will correct you in ordinary speech for pointing at it and saying vapour.

The word is mostly uncountable: some vapour, a lot of vapour. Scientists do say "vapours" when they mean several different kinds, especially chemical ones, and "toxic vapours" is a common phrase on warning labels.

A useful compound: vapour trail — the long white line a plane leaves behind it in the sky.