Define the coefficient of viscosity.

(N/A) The coefficient of viscosity,denoted by the Greek letter $\eta$ (eta),is a measure of a fluid's resistance to flow or deformation under shear stress.
According to Newton's law of viscosity,the viscous force $F$ acting between two layers of a fluid is given by $F = \eta A \frac{dv}{dx}$,where $A$ is the area of contact,and $\frac{dv}{dx}$ is the velocity gradient.
Rearranging the formula,we get $\eta = \frac{F}{A(dv/dx)}$.
Thus,the coefficient of viscosity is defined as the tangential force per unit area required to maintain a unit velocity gradient between two parallel layers of a fluid.
The $SI$ unit of the coefficient of viscosity is $\text{Pa} \cdot \text{s}$ or $\text{N} \cdot \text{s/m}^2$ (also known as Poiseuille,$\text{Pl}$).