Definition:F-Sigma Set

Definition
Let $T = \left({S, \tau}\right)$ be a topological space.

An $F_\sigma$ set (F-sigma set) is a set which can be written as a countable union of closed sets of $T$.

Also see

 * $G_\delta$ (G-Delta) Set
 * Complement of $F_\sigma$ Set is $G_\delta$ Set
 * Complement of $G_\delta$ Set is $F_\sigma$ Set

Linguistic Note
The name $F_\sigma$ originates from the French:
 * F for fermé, which is French for closed
 * $\sigma$ (Greek s) for somme, which is French for union.