Plain English Meaning
The height above the ground of the lowest cloud layer that covers more than half the sky. If the cloud cover is only scattered or thin, it doesn't count as a ceiling.
Full Definition
The height above the ground or water of the lowest layer of clouds or obscuring phenomena that is reported as 'broken,' 'overcast,' or 'obscuration,' and not classified as 'thin' or 'partial.'
Context Anchor
Pilots encounter ceiling in airport weather reports, forecasts, preflight weather briefings, and decisions about whether the weather is good enough for visual flying.