The axiom of foundation states that the membership relation is well founded, i.e. that any non-empty collection Y of sets has a member y which is disjoint from Y. This rules out sets which contain themselves (directly or indirectly).

In addition suitable contents:
[ ai ] [ an ] [ as ] [ at ] [ axiom ] [ b ] [ be ] [ ch ] [ co ] [ con ] [ de ] [ ec ] [ ed ] [ er ] [ es ] [ et ] [ fo ] [ fr ] [ h ] [ hat ] [ in ] [ int ] [ io ] [ ir ] [ is ] [ jo ] [ join ] [ la ] [ lv ] [ ly ] [ mp ] [ ms ] [ no ] [ om ] [ pt ] [ re ] [ relation ] [ ro ] [ rsh ] [ ru ] [ se ] [ set ] [ sh ] [ sj ] [ st ] [ state ] [ T ] [ th ] [ ve ] [ Y ]