The Collaborative International Dictionary of English v.0.48:

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]

The Collaborative International Dictionary of English v.0.48:

Ethylene \Eth"yl*ene\ ([e^]th"[i^]l*[=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.
   [1913 Webster]

   Ethylene series (Chem.), the series of unsaturated
      hydrocarbons of which ethylene is the type, and
      represented by the general formula CnH2n.
      [1913 Webster]