Boundary layer
From BAWiki
Definition
Area in flows between the wall and the "outer flow" in which the frictional forces are of the same magnitude as the inertial forces. This division uses the boundary layer theory according to which the external flow is treated as friction (potential flow), the boundary layer as frictionally but with the simplification that the velocity gradient can be neglected in the longitudinal direction relative to the transverse gradient in the flow. With the consequence that the pressure gradient can be neglected transversely to the flow, which means that the pressures boundary layer of the outer flow is impressed.