Viscosity
Viscosity () is the property of a fluid that resists movement between adjacent layers in the fluid. Viscosity can be thought of as a fluids resistance to an applied force ^{[1]}. Viscosity is similar to shear forces in solids. For example, in figure 1 the blue fluid has a lower viscosity than the orange fluid. It can be said that the orange fluid has a greater resistance to forces applied to it.
If you have two infinite in dimension parallel plates as in Figure 2 and the top plate is moving at a constant velocity while the bottom plate is stationary, the velocity of the fluid touching the top plate is the same as the upper plates velocity. However the velocity of the fluid layer directly under it is slightly slower. This pattern of decreasing velocity happens as you go down in the y direction. The fluid layer that is touching the bottom stationary plate has a velocity equal to 0. This force that is responsible for the change in velocity of the fluid is viscous drag. Newton's Law of Viscosity for a laminar flow is,

(1) 
where:
 is the force applied to the top plate
 is the area of each plate
 is the proportionality constant, viscosity of the fluid
 is the velocity in the z direction
 is the distance in the y direction
In equation (1), as approaches 0, in other words if the velocity does not change linearly with respect to y, then:

(2) 
In equation (2), which is applicable for SI units, is the shear stress or force per unit area () and is in units of N/m^{2}. If we wish to get the equation applicable for English units:

(3) 
Where lb_{m}*ft/lb_{f}*s^{2}.
In equation (3) the gravitational conversion factor () must be added in. Here is in units of lb_{f}/ft^{2}. The units of viscosity () in the cgs system are g/cm*s, also known as poise.
Useful Conversions When Working With Viscosity
cp poise g/cm*s
Pa*s N*s/m^{2} kg/m*s cp lb_{m}/ft*s 
References
 ↑ Geankoplis, Christie John. Transport Processes and Separation Process Principles. Pearson Education Inc, 2015. ISBN 013101367X. Book.