Vapour/ˈveɪpər/ Definition 1 of 2Pro Tip 1/3Change of State The term 'vapour' is used when a substance changes from solid or liquid to gas. The boiling kettle released a stream of vapour. Pro Tip 2/3Visible Gas 'Vapour' often refers to gas that you can see, like steam or fog. The cold air turned his breath into a vapour. Pro Tip 3/3Not Always Water While it's often water, 'vapour' can be any liquid or solid turned gas. Gasoline vapour can be highly flammable.