Fluid

In physics, a fluid is a substance that continually deforms (flows) under an applied shear stress. Fluids are a super set of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids. Although the term “fluid” includes both the liquid and gas phases, in common usage, “fluid” is often used as a synonym for “liquid”, with no implication that gas could also be present.