Let \(X\) be a topological space. A subset of \(X\) is an \(F_\sigma\) set if it is a countable union of closed sets, and a \(G_\delta\) set if it is a countable intersection of open sets.
In \(F_\sigma\), “F” stands for fermé, French for “closed”, and \(\sigma\) stands for a sum, that is, a union.