Source : Webster's Revised Unabridged Dictionary (1913)

Ethylene \Eth"yl*ene\ (-[=e]n), n. [From {Ethyl}.] (Chem.)
   A colorless, gaseous hydrocarbon, {C2H4}, forming an
   important ingredient of illuminating gas, and also obtained
   by the action of concentrated sulphuric acid in alcohol. It
   is an unsaturated compound and combines directly with
   chlorine and bromine to form oily liquids (Dutch liquid), --
   hence called {olefiant gas}. Called also {ethene}, {elayl},
   and formerly, {bicarbureted hydrogen}.

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}.