|Description: Axiom of Union. An axiom
of Zermelo-Fraenkel set theory. It states
that a set exists
that includes the union of a given set
i.e. the collection of all members of the members of . The
variant axun2 3931 states that the union itself exists. A
version with the
standard abbreviation for union is uniex2 3932. A version using class
notation is uniex 3933.
The union of a class df-uni 3367 should not be confused with the union of
two classes df-un 2832. Their relationship is shown in unipr 3380.