A gas is one of the phases of matter. Gases are, like liquids, fluids: they have the ability to flow and do not resist deformation. Unlike liquids, however, unconstrained gases do not occupy a fixed volume, but instead expand to fill whatever space they occupy. The kinetic energy in a gas is the greatest of the states of matter. Because of this increased kinetic energy, gas atoms and molecules tend to bounce off of one another, more so as the kinetic energy is increased.
Also see Ideal gas and Kinetic theory of gases
The term gas is also used to mean: