fluid

Fluid is a substance that deforms continuously under shear stress and thus flows. In physics, fluids include liquids and gases, which can alter their shape to match their containers. This property contrasts with solids, which resist deformation and can maintain a fixed shape.

Key properties of fluids include density, viscosity, and (for liquids) surface tension; gases are compressible, causing density and pressure to vary with height and temperature. Fluids may be approximated as incompressible in many engineering problems, and fluids can be Newtonian, with constant viscosity, or non-Newtonian, with viscosity depending on shear rate.

Fluid motion is described by fluid dynamics, often using the Navier–Stokes equations together with the continuity equation. Fluid statics (hydrostatics) deals with fluids at rest. Bernoulli's principle relates pressure, velocity, and height along a streamline in ideal, incompressible flow; real fluids require corrections for viscosity and turbulence.

Flow regimes range from laminar to turbulent, characterized by the Reynolds number. Phenomena such as buoyancy, capillarity, and boundary layers influence practical flows in pipes, airfoils, rivers, and blood vessels.

Fluids are central to many disciplines and technologies, including hydraulics, aerodynamics, meteorology, oceanography, and biomedical engineering, where understanding fluid behavior supports design and analysis of machines, environmental systems, and physiological processes.