Definition:G-Delta Set

Definition
Let $\left({X, \vartheta}\right)$ be a topological space.

A $G_\delta$ set (G-Delta set) is a set which can be written as a countable intersection of open sets of $X$.

Linguistic Note
The name $G_\delta$ originates from the German:
 * G for Gebiet, which is German for area or neighborhood (here used in the sense open set in this case;
 * $\delta$ (Greek d) for Durchschnitt, which is German for intersection.

Also see

 * $F_\sigma$ (F-Sigma) Set
 * Complement of F-Sigma Set is G-Delta Set