Fluid
A body whose parts yield to the slightest force when impressed, and by yielding are easily moved against each other. Fluids may be divided into elastic and non-elastic. Elastic fluids are those which may be compressed into a very small compass, but resume their former dimensions on removing the compressing force. These are distinguished as airs or gases. Non-elastic fluids are those which occupy the same bulk under all pressures, or, if compressible, it is only in a slight degree--as water, oils, etc. These are denominated liquids, except in the case of metals when melted. The physical nature, laws, and effects of non-elastic fluids at rest constitute the science of hydrostatics, and when in motion, of the science of hydraulics; those that relate to elastic fluids appertain to pneumatics. See SOLID.