Viscosity:

When the motion of a liquid over a fixed horizontal surface is slow and steady, i.e, when the motion of a liquid over a fixed horizontal surface is streamline, its contact with the fixed surface is stationary and the velocity of each layer increases with its distance from the fixed surface. If any two contiguous layers are considered, the upper layer moves faster than the lower layer and the lower layer moves slower than the upper layer, so that the upper layer tends to accelerate the motion of the lower layer, whereas the lower layer tends to retard the motion of the upper layer. Thus the two layers together tend to destroy the relative motion, as if there is a backward dragging force acting tangentially on the layers. Consequently, if a relative velocity is to be maintained between the two layers of a liquid an external force is required to overcome this backward drag.

This property of a liquid by virtue of which a liquid opposes the relative motion between its different layers is called viscosity or internal friction of a liquid.