Fluid. In a solid body the constituent particles never move far from a certain position of equilibrium. In a fluid the particles can move about with greater or less freedom from one part of the body to another. All liquids, vapours, and gases are therefore known as fluids. All fluids are perfectly elastic; but liquids are highly incompressible, while gases can easily be compressed. In every actual fluid there is more or less frictional resistance to the molecular motions; but it is often advantageous to consider theoretically the properties of frictionless, or, as they are called, 'perfect' fluids.
There is no sharp distinction between the solid state and the liquid state. Much depends upon external circumstances, such as temperature or the intensity of gravity. Some substances splinter under the action of sudden intense stress, while they flow like viscous liquids when exposed to long-continued gentle stress.