The displacement thickness is used to calculate the boundary layer's shape factor.

When the boundary layer separates, its displacement thickness increases sharply, which modifies the outside potential flow and pressure field.

The definition of the displacement thickness for compressible flow is based on mass flow rate:

First, the boundary layer adds to the effective thickness of the body, through the displacement thickness, hence increasing the pressure drag.

In practical aerodynamics, the displacement thickness essentially modifies the shape of a body immersed in a fluid to allow an inviscid solution.

The test result is evaluated against a standard boundary layer displacement thickness (BLDT) criterion, and a pass/fail assessment is made.

Displacement Thickness is an alternative definition stating that the boundary layer represents a deficit in mass flow compared to inviscid flow with slip at the wall.

The thickened boundary layer's displacement thickness changes the airfoil's effective shape, in particular it reduces its effective camber, which modifies the overall flow field so as to reduce the circulation and the lift.

The displacement thickness, δ or δ is the distance by which a surface would have to be moved in the direction perpendicular to its normal vector away from the reference plane in an inviscid fluid stream of velocity to give the same flow rate as occurs between the surface and the reference plane in a real fluid.