The state of matter distinguished from the solid and liquid states by: relatively low density and viscosity; relatively great expansion and contraction with changes in pressure and temperature; the ability to diffuse readily; and the spontaneous tendency to become distributed uniformly throughout any container WordNet 3.0 & Open English WordNet

Gas is one of the four fundamental states of matter. Wikipedia

One of the four main physical states of matter. Wikipedia Disambiguation

Fundamental state of matter in which constituent particles are widely separated with weak intermolecular bonds Wikidata

State of matter in which the matter concerned occupies the whole of its container irrespective of its quantity. OmegaWiki

A substance that continues to occupy in a continuous manner the whole of the space in which it is placed, however large or small this place is made, the temperature remaining constant. OmegaWiki

Matter in a state intermediate between liquid and plasma that can be contained only if it is fully surrounded by a solid (or in a bubble of liquid) (or held together by gravitational pull); it can condense into a liquid, or can (rarely) become a solid directly. Wiktionary

(uncountable, chemistry) state of matter. Wiktionary (translation)