An aeriform fluid; -- a term used at first by chemists as
   synonymous with air, but since restricted to fluids supposed to be
   permanently elastic, as oxygen, hydrogen, etc., in distinction from
   vapors, as steam, which become liquid on a reduction of temperature. In
   present usage, since all of the supposed permanent gases have been
   liquified by cold and pressure, the term has resumed nearly its
   original signification, and is applied to any substance in the elastic
   or aeriform state.