This page describes some parameters used to measure the properties of boundary layers. Consider a stationary body with a turbulent flow moving around it, like the semi-infinite flat plate with air flowing over the top of the plate. At the solid walls of the body the fluid satisfies a no-slip boundary condition and has zero velocity, but as you move away from the wall, the velocity of the flow asymptotically approaches the free stream to mean velocity.
Therefore it is impossible to define a sharp point at which the boundary layer becomes the free stream, yet this layer has a well-defined characteristic thickness. The parameters below provide a useful definition of this characteristic, measurable thickness.