Plain English Meaning
Ice forming and growing on the aircraft as it flies through cold, wet air. The longer it flies through those conditions, the more ice piles up.
Full Definition
The accumulation and build-up of ice on an aircraft surface caused by supercooled water droplets striking and freezing onto the airframe, propeller, windshield, or engine inlets during flight through visible moisture at or below freezing temperatures.
Context Anchor
Seen in instrument flying and weather discussions when describing icing on wings, propellers, , , and other aircraft surfaces.