When all the forces in any surface are such that the net force at a point comes to be zero then the object either do not move or in case if it moves it moves with a constant acceleration or velocity.
In this case the speed is neither gained nor lost, which means that there is no change it the motion of the object thus, the net force acting on this object is zero.
Viscosity refers to the speed at which a substance can flow (i.e. the thickness of the substance/its ability to flow). I think that's the closest the English language gets.
