I've been struggling with this concept for a very long time. I know about boundary layers in a very informal sense, such as in boundary layer separations at the trailing edge of airfoils and other objects immersed in flowing fluid. I also know about the boundary layer development in a pipe, as a plug flow develops into the parabolic distribution expected of a Newtonian fluid.
I've referred to Wikipedia, but I could not find a proper definition of what a boundary layer is:
In physics and fluid mechanics, a boundary layer is an important concept and refers to the layer of fluid in the immediate vicinity of a bounding surface where the effects of viscosity are significant.
I can expect the boundary layer connects points that have some common parameter, but what is it?