  Interface between two phases of matter are ubiquitous in nature and
technology. Determining the correct velocity condition at an interface is
essential for understanding and designing of flows over a surface. We
demonstrate that both the widely used no-slip and the Navier and Maxwell slip
boundary conditions do not capture the complete physics associated with complex
problems, such as spreading of liquids or corner flows. Hence, we present a
unified boundary condition that is applicable to a wide-range of flow problems.
