Ceiling Regulation
Ceiling means the height above the earth's surface of the lowest layer of clouds or obscuring phenomena that is reported as “broken”, “overcast”, or “obscuration”, and not classified as “thin” or “partial”.
Also defined in 1 more source
The height above the earth’s surface of the lowest layer of clouds, which is reported as broken or overcast, or the vertical visibility into an obscuration.
Guidance: Pilot's Handbook of Aeronautical Knowledge, p. G-6