The divalent groups formed from
alkanes by removal of two hydrogen atoms from the same carbon atom, the free valencies of which are part of a double bond, e.g.
(CH3)2C=
propan-2-ylidene.
Source:
PAC, 1995, 67, 1307
(Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995))
on page 1314
