vapour, n. & v. (US vapor)
1 moisture or another substance diffused or suspended in air, e.g. mist or smoke.
2 Physics a gaseous form of a normally liquid or solid substance (cf. GAS).
3 a medicinal agent for inhaling.
4 (in pl.) archaic a state of depression or melancholy thought to be caused by exhalations of vapour from the stomach.
1 rise as vapour.
2 make idle boasts or empty talk.
vapour density the density of a gas or vapour relative to hydrogen etc. vapour pressure the pressure of a vapour in contact with its liquid or solid form. vapour trail a trail of condensed water from an aircraft or rocket at high altitude, seen as a white streak against the sky.
vaporous adj. vaporously adv. vaporousness n. vapourer n. vapouring n. vapourish adj. vapoury adj.
ME f. OF vapour or L vapor steam, heat