Molecular entities with similar conjugate networks which have the same number of $\ce{\uppi\!\mbox{-}electrons}$,
e.g. cyclopentadienide
anion and pyrrole or benzene and pyridine.
Source: PAC, 1999,
71, 1919. (
Glossary of terms used in theoretical organic chemistry) on page 1947 [
Terms] [
Paper]