A G-delta set is a countable intersection of open subsets of a topological space. In a complete metric space these are precisely the topologically complete subspaces, by the G-delta criterion for topological completeness.
In the context of topology, a \( G_\delta \) set (pronounced "G delta set") is a subset of a topological space that can be expressed as a countable intersection of open sets.
New to topics? Read the docs here!