Plain English Meaning
The little upturned fins you see on the ends of airliner wings. They reduce the swirling air at the wingtip, which makes the wing more efficient and saves fuel.
Full Definition
Small, near-vertical extensions fitted to the tips of an aircraft's wings that reduce induced drag by limiting the spanwise airflow and weakening the wingtip vortices generated in flight.
Context Anchor
Seen in discussions of aircraft design, performance, fuel efficiency, and during preflight inspection of the wingtips.