Plain English Meaning
A special liquid sprayed on the airplane to melt off ice and snow before takeoff, and in some cases to keep new ice from forming for a short time.
Full Definition
A chemical liquid, typically a heated mixture of glycol and water, applied to an aircraft's surfaces before flight to remove existing frost, snow, or ice and, in some formulations, to provide short-term protection against re-accumulation.
Context Anchor
Seen in discussions of aircraft windshield ice protection and preflight removal of ice or frost.
Derivation
From 'de-' (to remove) and 'ice' -- literally a fluid that takes ice off. The 'de-' prefix comes from Latin, meaning to undo or reverse, which is exactly what the fluid does to any ice already on the aircraft.