Full Idea
Axiom of Union: ∀F ∃A ∀Y ∀x (x ∈ Y ∧ Y ∈ F → x ∈ A). That is, the union of a set (all the members of the members of the set) must also be a set.
Clarification
The 'F' is in a Gothic font in Kunen; it represents a 'family' of sets
Gist of Idea
Union: ∀F ∃A ∀Y ∀x (x ∈ Y ∧ Y ∈ F → x ∈ A)
Source
Kenneth Kunen (Set Theory [1980], §1.6)
Book Reference
Kunen,Kenneth: 'Set Theory: Introduction to Independence Proofs' [North-Holland 1980], p.12