Floor is a mathematical function, from ℜ to Z (the set of real numbers
to the set of integers). The floor of x is the greatest integer which is not
greater than x (i.e., the value of x, rounded down).
Basically, start at x on the number line, and if it's not already an integer
move in the negative direction until you hit an integer.
The mathematical notation for taking the floor of x is to surround it by
vertical bars, like the absolute value notation, except with short horizontal stems
joined to the bottom of the bars and pointing inwards (like [x] with the
top stems missing). The HTML tags are ⌊ and ⌋ and on your browser
they look like this:
Compare with ceiling.