ceiling

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 ⌈ and ⌉ 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.

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.

© Webster 1913.