A Fluid:

A fluid is defined as a substance that continually deforms (flows) under an applied shear stress regardless of how small the applied stress. All liquids and all gases are fluids. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids. The term "fluid" is often used as being synonymous with "liquid". This can be erroneous and sometimes clearly inappropriate - such as when referring to a liquid which does not or should not involve the gaseous state. "Brake fluid" is hydraulic oil which will not perform its required function if gas is present. The medical profession relies on the term "fluids" in dietary references ("take plenty of fluids") where the presence of gases is irrelevant or even possibly dangerous.Liquids form a free surface (that is, a surface not created by the container) while gases do not. The distinction between solids and fluid is not entirely obvious. The distinction is made by evaluating the viscosity of the substance. Silly Putty can be considered either a solid or a fluid, depending on the time period over which it is observed.Fluids display such properties as:not resisting deformation, or resisting it only lightly (viscosity), andthe ability to flow (also described as the ability to take on the shape of the container).