A <fluid> is a material that flows under sustained <shear stress> and cannot maintain a static <shear stress> in its fluid state. <Liquids> and <gases> are the usual examples. A <Newtonian fluid> has <deviatoric stress> proportional to its <rate-of-strain tensor>, whereas more general fluid <constitutive equations> can be nonlinear or have memory. In a <porous medium>, the <fluid> occupies connected voids within a load-bearing <solid> framework. The important distinction is the response to a continuing tangential <force>: an ordinary elastic <solid> can settle into a fixed <deformation>, while an ordinary <fluid> continues to deform. A <fluid> can nevertheless be at rest when no unbalanced <shear stress> drives it. https://ocw.mit.edu/courses/12-800-fluid-dynamics-of-the-atmosphere-and-ocean-fall-2004/7bb9e945405534b36776cbe404a8f839_lecture3.pdf[MIT, Fluid Dynamics of the Atmosphere and Ocean, lecture 3, pp. 4–5]{ref}

For a worked example, place a <Newtonian fluid> between parallel plates separated by $h=1\,\mathrm{mm}$, keep the lower plate fixed and move the upper one at $U=0.10\,\mathrm{m\,s^{-1}}$. With <no-slip boundary conditions>, <steady flow>, no imposed <pressure gradient> and negligible edge effects, <Couette flow> has $u(y)=Uy/h$. For <dynamic viscosity> $\mu=0.001\,\mathrm{Pa\,s}$, the <shear stress> magnitude is
$$
\tau=\mu\frac{du}{dy}=\mu\frac{U}{h}=0.10\,\mathrm{Pa}.
$$
The <velocity gradient> describes neighbouring layers sliding at different speeds. Doubling $U$ doubles the required <shear stress>; maintaining the same <velocity> keeps producing <deformation>, rather than fixing a final shape. This gives <viscosity> a concrete meaning: resistance to a rate of motion, not a prohibition on motion.


 Fluid (source code)