Source : Webster's Revised Unabridged Dictionary (1913)

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}.