To round a real number down is to take the greatest integer which is not
greater than the specified number, i.e. if the number is not already an integer move in
the negative direction on the number line (usually left) until you hit an integer.
This operation is also called taking the floor of a number. For example, to round down 3
gives 3, to round down 3.5 gives 3, to round down 7.2 gives 7, to round down -0.2 gives -1,
to round down -99.7 gives -100.

Compare to round up and round off.