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⌉.