Newtonian fluid

fluid in which the viscous stresses arising from its flow, at every point, are linearly proportional to the local strain rate—the rate of change of its deformation over time; the simplest mathematical model of fluid that account for viscosity

A Newtonian fluid is a fluid, where the ratio between shear stress changes linearly in proportion to the stress it is exposed to. This proportion is known as viscosity. Note that the inverse is not true; there are fluids with constant viscosity, that are Non-Newtonian fluids: one example is the Boger fluid.