The Collaborative International Dictionary
ethidene
Ethylidene \E*thyl"i*dene\ (Chem.) An unsymmetrical, divalent, hydrocarbon radical, C2H4 metameric with ethylene but written thus, CH3.CH= to distinguish it from the symmetrical ethylene, CH2=CH2. Its compounds are derived from aldehyde. Formerly called also ethidene.
Wiktionary
ethidene
n. (context obsolete English) ethylidene