G-delta set

Summary

A $G_\delta$ set in a topological space is an intersection of countably many open sets. The name comes from the German words for open set and intersection. In a metric space every closed set is $G_\delta$, and $G_\delta$ sets are the next step after open and closed sets in the Borel hierarchy. The complements of $G_\delta$ sets are the F-sigma sets.