The hydrocarbon fulvene and its derivatives formed by substitution (and by extension,
analogues formed by replacement of one or more carbon atoms of the fulvene skeleton
by a heteroatom).
Source:
PAC, 1995, 67, 1307
(Glossary of class names of organic compounds and reactivity intermediates based on
structure (IUPAC Recommendations 1995))
on page 1336