|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 4451 states that the union itself exists. A
version with the
standard abbreviation for union is uniex2 4452. A version using class
notation is uniex 4453.
The union of a class df-uni 3769 should not be confused with the union of
two classes df-un 3099. Their relationship is shown in unipr 3782.
(Contributed by NM, 23-Dec-1993.)