Plain English Meaning
When a pilot accidentally flies into conditions where ice starts forming on the aircraft, even though they did not plan to be there.
Full Definition
An unplanned encounter with in-flight icing conditions, in which an aircraft accumulates ice on its surfaces while operating in visible moisture at temperatures at or below freezing, despite the pilot not having intended to enter such conditions.
Context Anchor
Seen in instrument flying, weather planning, and in-flight decision-making when a pilot enters clouds or precipitation near freezing temperatures and must leave the icing conditions.