- For other meanings see gas (disambiguation).
A gas is one of the phases of matter. Like liquids, gases are fluids: they have the ability to flow and do not resist deformation, although they do have viscosity. 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 second greatest of the states of matter (after plasma). Because of this increased kinetic energy, gas atoms and molecules tend to bounce off of any containing surface and off one another, the more powerfully as the kinetic energy is increased. A common misconception is that the collisions of the molecules with each other is essential to explain gas pressure, but in fact their random velocities are sufficient to define that quantity. Mutual collisions are important only for establishing the Maxwell-Boltzmann distribution.
The word "gas" was probably coined by a Flemish chemist as a re-spelling of his pronunciation of the Greek word chaos.