Dense

A set $A$ in a First-Countable Space is dense in $B$ if $B=A\cup L$, where $L$ is the limit of sequences of elements of $A$. For example, the rational numbers are dense in the reals. In general, a Subset $A$ of $X$ is dense if its Closure $\mathop{\rm cl}(A) = X$.

See also Closure, Density, Derived Set, Perfect Set