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}.
[1913 Webster]