Since the fluid velocity in the boundary layer smoothly joins that of the outer flow, there is no obvious demarcation of the boundary layer’s edge. Thus, a variety of thickness definitions are used to define a boundary layer’s character

The three most common thickness definitions are described here

• The overall boundary-layer thickness is defined by $\delta_{99}$, the distance from the wall at which the local streamwise velocity reaches 99% of the edge (free-stream) speed \(u(x,\delta_{99}) = 0.99\,U_e(x)\). It is used mainly as a conceptual thickness, it is hard to measure accurately and the choice of 99% (vs. 95%, 98%, 99.5%, etc.) is arbitrary, so its physical significance is somewhat subjective. This is the commonly used engineering nominal thickness of the boundary layer
• A second measure of the boundary-layer thickness, and one in which there is no arbitrariness, is the displacement thickness, which is commonly denoted $\delta^*(x)$ or $\delta_1$
• A third measure of the boundary-layer thickness is the momentum thickness $\theta$ or $\delta_2$. It is defined such that $\rho U^2 \theta$ is the momentum loss in the actual flow because of the presence of the boundary layer