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]