CH2:CH

from The Collaborative International Dictionary of English v.0.48

Ethenyl \Eth"e*nyl\, n. [Ethene + -yl.] (Chem.)
   (a) A trivalent hydrocarbon radical, {CH3.C}.
   (b) A univalent hydrocarbon radical of the ethylene series,
       {CH2:CH}; -- called also {vinyl}. See {Vinyl}.
       [1913 Webster]