Definition:Nowhere Dense

Definition
Let $T$ be a topological space.

Let $H \subseteq T$.

Then $H$ is nowhere dense in $T$ iff:
 * $\left({H^-}\right)^\circ = \varnothing$

where $H^-$ denotes the closure of $H$ and $H^\circ$ denotes its interior.

That is, $H$ is nowhere dense in $T$ iff the interior of its closure is empty, i.e. it is "all boundary".

Alternative Definition
A set $H$ is nowhere dense in $T$ iff $H^-$ contains no open set of $T$ which is not empty.

These definitions can be seen to be directly equivalent from the definition of interior as the union of all subsets of $H$ open in $T$.

Example
The set $\left\{{\dfrac 1 n: n \in \N}\right\}$ is nowhere dense in $\R$.

Also see

 * Everywhere dense
 * Dense-in-itself