CH2=CH2

from 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]