Definition:Everywhere Dense/Real Numbers

Definition
Let $S$ be a subset of the real numbers.

Then $S$ is (everywhere) dense in $\R$ :
 * $\forall x \in \R : \forall \epsilon \in \R : \epsilon > 0: \exists s \in S: x - \epsilon < s < x + \epsilon$.

That is, in every neighborhood of every real number lies an element of $S$.