Plain English Meaning
Chunks of ice that fall from thunderstorms. They form when strong rising air inside the storm lifts water drops high enough to freeze, then keeps tossing them up and down until they grow heavy enough to fall.
Full Definition
A form of solid precipitation consisting of balls or irregular lumps of ice, formed in strong thunderstorm updrafts that repeatedly carry water droplets above the freezing level, where they accumulate successive layers of ice before falling to the ground.
Context Anchor
Seen in weather briefings, thunderstorm forecasts, pilot reports, and decisions about avoiding storm cells.
Derivation
From Old English 'hagol,' meaning small stones of ice falling from the sky. The everyday meaning and the aviation meaning are the same — the term has not drifted.