In mathematics, particularly topology, a Gδ space is a topological space in which closed sets are in a way ‘separated’ from their complements using only countably many open sets.
In fact normal Gδ spaces are referred to as perfectly normal spaces, and satisfy the strongest of separation axioms.
A countable intersection of open sets in a topological space is called a Gδ set.
A topological space X is called a Gδ space[2] if every closed subset of X is a Gδ set.
Dually and equivalently, a Gδ space is a space in which every open set is an Fσ set.