Axiom:Axiom of Foundation

For all non-null sets, there is an element of the set that shares no member with the set.

$$\forall S: \lnot \left({S = z: \forall y: \left({\lnot \left({y \in z}\right)}\right)}\right) \Longrightarrow \exists x \in S: \lnot \left({\exists w: w \in S \land w \in x}\right)$$

Otherwise known as the Axiom of Regularity.

It can also be stated as:

"A set contains no infinitely descending (membership) sequence."

"A set contains a (membership) minimal element."