What is the boundary layer and why is it important?

The boundary layer is the thin region of air adjacent to the aircraft surface where viscous effects dominate and velocity transitions from zero at the surface (no-slip condition) to the freestream velocity. It can be laminar (smooth, low drag) or turbulent (higher drag but more resistant to separation). Understanding boundary layer behavior is critical for predicting skin friction drag, flow separation, heat transfer, and designing effective flow control devices.