Fluid.
Synonyms:
gas, liquid.
A fluid is a substance that, like air or water, yields to any force that tends to change its form; a liquid is a body in that state in which the particles move freely among themselves, but remain in one mass, keeping the same volume, but taking always the form of the containing vessel; a liquid is an inelastic fluid; a gas is an elastic fluid that tends to expand to the utmost limits of the containing space. All liquids are fluids, but not all fluids are liquids; air and all the gases are fluids, but they are not liquids under ordinary circumstances, tho capable of being reduced to a liquid form by special means, as by cold and pressure. Water at the ordinary temperature is at once a fluid and a liquid.