Plain English Meaning

Air rubbing against the outside of the airplane as it flies. Even though air seems thin and slippery, it sticks slightly to every surface, and that stickiness creates resistance.

Full Definition

A type of parasite drag caused by the resistance of air molecules dragging along the surface of the airframe as the airplane moves through the air. It results from the viscosity of air interacting with the aircraft's outer skin, particularly within the thin layer of air immediately adjacent to the surface known as the boundary layer.

Context Anchor

Seen in aerodynamics and performance discussions, especially when comparing clean, smooth surfaces with dirty, rough, or ones.