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.

Source: Webster's Revised Unabridged Dictionary (1913)