A CyclicDependency in a mathematical type hierarchy. Most math seems to follow an AcyclicDependenciesPrinciple.

''RussellParadox is an example of what the poster means; a set whose definition refers to itself.  Many SetTheory''''''s (there are several different ones) exclude such things in order to avoid the paradox.''

''Fkk (see GoedelsIncompletenessTheorem) is another example of a StrangeLoop.  Interestingly enough, most mathematicians still consider Fkk a valid sentence in FirstOrderLogic, even though Goedels theorem is kindasorta analogous to RussellParadox...''