Recall that, given any subset $A$ of any topological space, a point $x$ belongs to the closure of $A$ if and only if $x$ is a limit point of at least one net whose elements belong to $A$.

A topological space is Frechet–Uryson (or Frechet–Urysohn) if a point $x$ of the closure of any given subset $A$ of $X$ is a limit point of at least one sequence whose elements belong to $A$.