Separable space

In topology, a **separable space** is a type of topological space that contains a countable dense subset. More formally, a topological space \( X \) is said to be separable if there exists a countable subset \( D \subseteq X \) such that the closure of \( D \) is equal to \( X \). This means that every point in \( X \) can be approximated arbitrarily closely by points from \( D \).