Definition
A system that prevents ice from forming on the leading edges of airfoil surfaces such as wings, horizontal stabilizers, and vertical stabilizers. On turbine-powered airplanes, it is most commonly supplied by hot bleed air routed from the engine compressor through ducts inside the leading edges, keeping those surfaces warm enough that ice cannot adhere.
Plain English
A system that stops ice from forming on the wings and tail by keeping their front edges warm, usually with hot air taken from the engine.
Context Anchor
You will see this term in turbine airplane systems, cold-weather operations, and checklists for flight near clouds, rain, or snow when temperatures are near or below freezing.
Derivation
Airfoil comes from 'air' plus 'foil' (a thin shaped surface), meaning a wing-shaped surface that produces lift. 'Anti' is from Greek for 'against,' and 'ice' is the everyday word. Together it literally means 'against ice on the wing-shaped surface' — a system that works to prevent ice rather than remove it.
Why Pilots Care
Ice changes airfoil shape, reduces lift, and raises stall speed; the system keeps control and performance normal in icing.
Intuition Check
Do not read anti-ice as the same thing as deice. Anti-ice is used to help prevent ice from forming; deice is used to remove ice that has already formed.
Example Sentence 1
Before climbing into the cloud layer, the crew turned on the airfoil anti-ice to keep the wing leading edges clear of ice.
Example Sentence 2
With airfoil anti-ice active, the wings kept full lift even in light rime.