# Newtonian fluid

A **Newtonian fluid** is a fluid in which shear stress is proportional to the velocity gradient, perpendicular to the plane of shear. The constant of proportionality is known as the viscosity.

The motion of Newtonian fluids is described by the Navier-Stokes equations.

See also: Non-Newtonian fluid | Poiseuille's law | Viscosity