Definition
The thin layer of air immediately adjacent to the surface of the wing (or other airframe surface) where the airflow is slowed by friction with the surface. Within this layer, air velocity changes from zero at the surface to nearly the free-stream velocity at the layer's outer edge. The boundary layer can be either laminar (smooth, orderly flow) or turbulent (mixed, chaotic flow), and its behavior strongly affects lift, drag, and stall characteristics.
Plain English
It's the very thin film of air clinging to the wing's surface that gets dragged along and slowed down by the wing as it moves through the air. How this thin layer behaves has a big effect on how the wing produces lift and when it stalls.
Context Anchor
Seen in stall and airflow discussions, especially when explaining how smooth airflow over a wing can separate as angle of attack increases.
Derivation
From 'boundary' (the edge or limit between two things) and 'layer' (a sheet of material). It's the layer of air at the boundary between the wing's surface and the open airflow beyond it.
Why Pilots Care
Boundary layer separation triggers the stall, directly affecting lift, drag, and how safely an aircraft can be flown near critical angles of attack.
Analogy
Picture moving your hand slowly through water. A very thin film of water seems to travel with your hand while the water further away stays still. The wing carries a similar thin film of air with it as it flies.
Grounding Statement
Picture the air touching the wing being dragged along by the surface, while the air just above it keeps moving faster.
Intuition Check
Do not think of the boundary layer as a hard line or a visible layer painted on the wing. It is a thin moving region of air next to the surface, slowed by contact with that surface.
Example Sentence 1
Frost on the wings disrupts the boundary layer, which can cause the wing to stall at a lower angle of attack than normal.
Example Sentence 2
The pilot lowered the flaps to energize the boundary layer and keep airflow attached at a higher angle of attack.