Gas (n.) : 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.
Gas (n.) : A complex mixture of gases, of which the most important constituents are marsh gas, olefiant gas, and hydrogen, artificially produced by the destructive distillation of gas coal, or sometimes of peat, wood, oil, resin, etc. It gives a brilliant light when burned, and is the common gas used for illuminating purposes.
Gas (n.) : Laughing gas.
Gas (n.) : Any irrespirable aeriform fluid.