visible particles of moisture floating in the air, as fog, mist, or steam
any cloudy or imperceptible exhalation, as smoke or noxious fumes
the gaseous form of any substance which is a liquid or a solid at room temperature
any substance vaporized for use in machinery, medical therapy, etc.
a mixture of such a vaporized substance with air, as the explosive mixture in an automotive cylinder
NOW RARE anything insubstantial or worthless