are easily compressible fluids that expand to fill their containers, and expand indefinitely in the absence of boundary restraints, As a result of their compressibility, gases may have large change in volume and density. However, under conditions of relatively constant pressure and temperature, gases will have relatively constant volumes.

formless fluids that expand to occupy the space or enclosure in which they are confined. Examples are nitrogen, oxygen and carbon dioxide of the air.

A aeriform fluid consisting of fine particles as suspended in air, having an indefinite expansion or shape, gaseous particles will expand by their same fraction for every degree that the temperature rises.

are fluids that have no shape and expand to fill the shape of the space in which they are contained.

Have no definite volume and no definite shape.

The vapor state of matter; nonelastic fluids in which the molecules are in free movement and their mean positions far apart. Gases tend to expand indefinitely, to diffuse and mix readily with other gases, to have definite relations of volume, temperature, and pressure, and to condense or liquefy at low temperatures or under sufficient pressure. (Grant & Hackh's Chemical Dictionary, 5th ed)

Substances with the following properties: 1) Indefinite and uniform expansion 2) Indefinite shape or volume 3) Compressibility 4) Low density 5) Mixability or fusion

Formless fluids which tend to occupy an entire space uniformly at ordinary temperatures and pressures.