Heterocyclic compounds formally derived from arenes by replacement of one or more methine ($\ce{–C=}$) and/or vinylene ($\ce{–CH=CH\bond{–}}$) groups by trivalent or divalent heteroatoms, respectively, in such a way as to maintain the continuous $\ce{\uppi\!\mbox{-}electron}$ system characteristic of aromatic systems and a number of out-of-plane $\ce{\uppi\!\mbox{-}electrons}$ corresponding to the Hückel rule (\(4\ n+2\)); an alternative term is hetarenes.