Plain English Meaning

Air doesn't just flow straight back over a wing. Some of it slides sideways along the wing because the pressure underneath is higher than the pressure on top, and that high-pressure air leaks around the tip toward the low-pressure side.

Full Definition

Airflow that moves along the length of a wing — from root to tip on the lower surface, and from tip to root on the upper surface — rather than straight back from leading edge to trailing edge. It is caused by the pressure difference between the high-pressure air below the wing and the low-pressure air above it, with the air spilling around the wingtip from bottom to top.

Context Anchor

Seen in discussions of induced drag, , and how a wing makes .