When a fluid flows over a solid boundary, its velocity matches that of the boundary due to no slip. Near the boundary, a thin region called the boundary layer forms where velocity increases from zero at the boundary to the free stream value. Within this layer, a velocity gradient and shear stress exist due to the changing velocity from the boundary to outside flow. The boundary layer thickness is where velocity reaches 99% of the free stream value, while displacement thickness represents the boundary shift needed to compensate for reduced flow due to the layer. Boundary layer separation can occur where flow detaches from the boundary.