BOUNDARY LAYER CONCEPT IN THE STUDY OF FLUID FLOW

 The velocity within the boundary layer increases from zero at the boundary surface to the velocity of the main stream asymptotically. Therefore the thickness of the boundary layer is arbitrarily defined as that distance from the boundary in which the velocity reaches 99 per cent of the velocity of the velocity of the free stream (u=99U). It is denoted by the symbol. This definition however gives an approximate value of the boundary layer thickness and hence  is generally termed as  

nominal thickness of the boundary layer.

Full details in this file BOUNDARY LAYER CONCEPT IN THE STUDY OF FLUID FLOW