Gas is one of the states of matter, in which the molecules move about unconstrained by each other and will expand into any available space. Any such fluid used as an anesthetic, as nitrous oxide; any such combustible fluid used as fuel. 
Gas is short for gasoline; a volatile, flammable liquid mixture of hydrocarbons, obtained from petroleum, and used as fuel for internal-combustion engines, as a solvent, etc. 
Our atmosphere is mostly composed of gases.
Many gases are colorless, however, some are not. For instance, chlorine gas is greenish-yellow, and N2O4 is reddish-brown.
Sulfur dioxide and other volcanic gases rise from the Pu`u `O`o vent on Kilauea Volcano, Hawaii.