1927 Chem. Abstr. XXI. 4576/1-idene added to any radical usually means a double bond at point of attachment. 
1966 Nomencl. Org. Chem. (I.U.P.A.C.) (ed. 2) A. 16Bivalent and trivalent radicals derived from univalent acyclic hydrocarbon radicals whose authorized names end in ‘-yl’ by removal of one or two hydrogen atoms from the carbon atom with the free valences are named by adding ‘-idene’ or ‘-idyne’, respectively, to the name of the corresponding univalent radical. 
ORIGIN: from ethyl)idene 
-i·dene
\əˌdēn\  noun suffix 
(-s) 
Etymology: International Scientific Vocabulary -ide (I) + -ene 
 : radical having two valence bonds at the point of attachment 
< ethylidene CH3CH< >
< ethidene >
— compare -ylidene
< ethylidene CH3CH< >
< ethidene >
— compare -ylidene