Full Idea
Axiom of Set Existence: ∃x (x = x). This says our universe is non-void. Under most developments of formal logic, this is derivable from the logical axioms and thus redundant, but we do so for emphasis.
Gist of Idea
Set Existence: ∃x (x = x)
Source
Kenneth Kunen (Set Theory [1980], §1.5)
Book Reference
Kunen,Kenneth: 'Set Theory: Introduction to Independence Proofs' [North-Holland 1980], p.10