What is a boundary layer?

The boundary layer is defined as the region of fluid flow in the vicinity of a solid surface, such as an airfoil, where the effects of viscosity are significant. In this layer, the velocity of the airflow changes from zero at the surface—due to the no-slip condition—to the free stream velocity as you move away from the surface.

This phenomenon is a result of viscous forces acting on the fluid, which cause the air molecules closest to the surface to adhere to it, creating a gradient in the velocity of the air. The characteristic of local airflow retardation is essential in understanding various aerodynamic effects, such as drag and lift.

The other definitions do not accurately capture the essence of the boundary layer. The airflow that is described in the other choices lacks the crucial point about viscosity and its impact on the velocity gradient, which is fundamental to understanding why the boundary layer develops and behaves as it does in the presence of a solid surface.