In physics, a fluid is a substance that continually deforms (flows) under an applied shear stress. The characteristic that distinguishes a fluid from a solid is its inability to resist deformation under an applied shear stress (a tangential force per unit area). Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids.
By definition, a solid material is rigid. For example, if one were to impose a shear stress on a solid block of steel, the block would not begin to change shape until an extreme amount of stress has been applied. To be more exact, when a shear stress is first applied to a rigid material it deforms slightly, but then springs back to its original shape when the stress is relieved.
Phase diagram of water.
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A plastic material, such as clay, also possess some degree of rigidity. However, the critical shear stress above which it yields is relatively small, and once this stress is exceeded the material deforms continuously and irreversibly, and does not recover
its original shape when the stress is relieved.
By definition, a fluid material possesses no rigidity at all. For example, if one were to impose a shear stress on a fluid element, the fluid element deforms, because it is unable to withstand any tendency of an applied shear stress to change its shape. Furthermore, the more stress that is applied, the more the fluid element will deform. This provides us with a characterizing feature of liquids (and gases—fluids, in general) that distinguishes them from other forms of matter, and we can thus give a formal definition.
There are two types of fluid: liquids and gases. The most important difference between these two types of fluid is in their relative compressibility. Gases can be compressed much more easily than liquids. Consequently, any motion that involves significant pressure variations is generally accompanied by much larger changes in mass density in the case of a gas than in the case of a liquid.