The height above the ground or water of the base of the lowest layer of cloud below 6000 meters (20,000 feet) covering more than half the sky.
United Kingdom 
In relation to an aerodrome, the vertical distance from the elevation of the aerodrome to the lowest part of any cloud visible from the aerodrome which is sufficient to obscure more than one half of the sky so visible.
A ceiling or cloud 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. A ceiling listed as "unlimited" means that the sky is clear or is free of any substantial cloud cover.
Ceiling is reported as part of the METAR (METeorological Aviation Report) used for flight planning by pilots worldwide.
See also 
- Visual flight rules (VFR)
- UK Air Navigation Order
- Section 1.1 14 CFR Part 1 - Definitions - Federal Air Regulations