There is no general accepted or standard notation for the Zermelo-Fraenkel Axioms. This axiom is called “Axiom of the Sum Set: For any X there exists a set Y the union of all elements of X. (also called Axiom of Union)” It is sometimes symbolized as

. You can find a good discussion, with examples, inELEMENTS OF SET THEORYby H.B. Enderton.