ICAO

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.

USA

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

METAR

METAR is a format for reporting weather information. A METAR weather report is predominantly used by pilots in fulfillment of a part of a pre-flight weather briefing, and by meteorologists, who use aggregated METAR information to assist in weather forecasting....

 (METeorological Aviation Report) used for flight planning by pilots worldwide.

See also

• Visual flight rules
  Visual flight rules
  Visual flight rules are a set of regulations which allow a pilot to operate an aircraft in weather conditions generally clear enough to allow the pilot to see where the aircraft is going. Specifically, the weather must be better than basic VFR weather minimums, as specified in the rules of the...
  
   (VFR)
• METAR
  METAR
  METAR is a format for reporting weather information. A METAR weather report is predominantly used by pilots in fulfillment of a part of a pre-flight weather briefing, and by meteorologists, who use aggregated METAR information to assist in weather forecasting....
  
  (METeorological Aviation Report)