In
Pilot's Jargon, the ceiling is a measure of current
visibility. It is expressed in
altitude, and is the height of the bottom of the lowest cloud
strata present, which (from the ground) would appear to be a 'ceiling.' When the
sky is completely clear, the ceiling is said to be
unlimited.
When used in the context of particular aircraft, the term refers to the maximum altitude attainable by that aircraft under specified conditions.