Definition:Floor Function

Let $$x \in \mathbb{R}$$. Then $$\left \lfloor {x} \right \rfloor$$ is defined as:

$$\left \lfloor {} \right \rfloor: \mathbb{R} \to \mathbb{Z}: \left \lfloor {x} \right \rfloor = \sup \left({\left\{{m \in \mathbb{Z}: m \le x}\right\}}\right)$$

That is, $$\left \lfloor {x} \right \rfloor$$ is the greatest integer less than or equal to $$x$$.

Notation
The notation given here is as popularised by Knuth, Compare the notation for the ceiling function, which in the context of discrete mathematics is used almost as much.

The older and more usual symbol for this is $$\left [{x} \right ]$$, and it is often referred to as the integer part of $$x$$, but there are already enough uses to which square brackets are put.