| Thin layer of fluid surrounding a body. It can be laminar or turbulent. A laminar boundary layer is one in which the fluid velocity is well-ordered and regular throughout the boundary layer. The speed of the fluid is taken to be zero at the surface of the body. A turbulent boundary layer is one in which the fluid velocity shows a great deal of randomness throughout the layer. A turbulent boundary layer is responsible for increased drag on the body, since the mean velocity profile of the boundary layer is 'fuller' than that of a laminar boundary layer, resulting in increased shear stress at the body's surface. Laminar to turbulent transition occurs on, for example, the raised red stitches on a baseball. This drag transition is responsible for a certain portion of the behavior of knuckleballs, curve balls, sliders, etc.
Aircraft whose value is determined by the cost of operation per passenger-mile typically require a great deal of thought and planning be put into where, when, and if boundary layer transition occurs on their wings. When a boundary layer is forced to turn through an excessively high angle on the surface of the body, it detaches and forms the boundary of a wake. On an airfoil, boundary layer detachment results in a stall. A laminar boundary layer will detach more readily than a turbulent one. This is why a smooth ping pong ball curves quite a lot when struck hard through the air. It would curve less if it had raised stitches, if memory serves.
If the fluid in question is air, the boundary layer is typically the only location in a flowfield where fluid viscosity cannot be neglected. Many calculations ignore the effects of the boundary layer since it is generally very thin with respect to the size of the body it surrounds. The boundary layer is responsible for the bulk of the heat transfer which occurs at the surface of a hypersonic vehicle (like the space shuttle).
In a pipe, the boundary layer's thickness grows until the thickness of the boundary layer equals the radius of the pipe. Using this knowledge, and formulae for the speed of boundary layer growth, calculations of pipe pressure loss per linear foot are possible.
Basically, a critical concept in fluid mechanics. | 
