Dipolar five- (possibly six-) membered
heterocyclic compounds in which both the negative and the positive charge are delocalized, for which a totally
covalent structure cannot be written, and which cannot be represented satisfactorily
by any one polar structure. The formal positive charge is associated with the ring
atoms, and the formal negative charge is associated with ring atoms or an exocyclic
nitrogen or chalcogen atom. Mesoionic compounds are a subclass of
betaines.
Source:
PAC, 1995, 67, 1307
(Glossary of class names of organic compounds and reactivity intermediates based on
structure (IUPAC Recommendations 1995))
on page 1349