Definition
A wingtip vortex is a swirling, tube-shaped mass of air that trails behind each wingtip of an aircraft in flight. It forms because high-pressure air beneath the wing spills around the wingtip into the lower-pressure air above the wing, creating a rotating flow that streams aft. Wingtip vortices are strongest when the aircraft is heavy, slow, and in a clean configuration (gear and flaps up).
Plain English
When a wing produces lift, air under the wing pushes up and around the tip into the area above the wing. That sideways spill gets dragged backward as the aircraft moves forward, leaving a pair of spinning air tubes behind the wingtips.
Context Anchor
Seen in discussions of drag, lift, slow flight, takeoff, landing, and wake turbulence behind other aircraft.
Derivation
Vortex comes from Latin vortex/vertex, meaning a whirl or whirlpool. The wingtip part simply names where the whirl forms. The image of a whirlpool is exactly right -- a rotating column of fluid, just made of air instead of water.
Why Pilots Care
The vortices persist as strong wake turbulence that can roll or pitch a following aircraft, requiring increased separation behind heavier airplanes.
Analogy
Think of dragging your hand through still water. Two small whirlpools form off your fingertips and trail behind. A wing does the same thing in air, just on a much larger scale.
Intuition Check
A wingtip vortex is not just a visible swirl at the very end of the wing. It is a real rotating flow of air trailing behind the aircraft, and it can affect another aircraft even when you cannot see it.
Example Sentence 1
The tower instructed the small trainer to extend its downwind leg to allow the wingtip vortices from the departing airliner to dissipate.
Example Sentence 2
Controllers increased spacing behind the heavy aircraft because of its strong wingtip vortices.