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.

Ceiling is a mathematical function, from ℜ to Z (the set of real numbers to the set of integers). The ceiling of x is the least integer which is not less than x (i.e., the value of x, rounded up). Basically, start at x on the number line, and if it's not already an integer move in the positive direction until you hit an integer. (The equivalent C function is called ceil.)

The mathematical notation for taking the ceiling of x is to surround it by vertical bars, like the absolute value notation, except with short horizontal stems joined to the top of the bars and pointing inwards (like [x] with the bottom stems missing). The HTML tags are &lceil; and &rceil; and on your browser they look like this:
x⌉.

Ceil"ing, n. [See Cell, v. t.]

1. Arch. (a)

The inside lining of a room overhead; the under side of the floor above; the upper surface opposite to the floor.

(b)

The lining or finishing of any wall or other surface, with plaster, thin boards, etc.; also, the work when done.

2. Naut.

The inner planking of a vessel.

Camp ceiling. See under Camp. -- Ceiling boards, Thin narrow boards used to ceil with.