A boundary condition used in fluid mechanics. When a liquid flows over the surface of a solid, and layers of liquid close to the solid are assumed to be stationary relative to the solid, this is called a no-slip boundary condition. Such an assumption is a good approximation for macroscopic flow but is not accurate at the molecular scale. When this assumption is not made a slip boundary condition occurs. Boundary slip causes the viscosity at the interface to be different from the viscosity in the bulk of the liquid.