The Collaborative International Dictionary of English v.0.48:

Methylene \Meth"yl*ene\, n. [F. m['e]thyl[`e]ne, from Gr. ? wine
   + ? wood; -- a word coined to correspond to the name wood
   spirit.] (Chem.)
   A divalent hydrocarbon radical, -CH2-, not known in the
   free state, but regarded as an essential residue and
   component of certain derivatives of methane; as, methylene
   bromide, CH2Br2; -- formerly called also methene.
   [1913 Webster]

   Methylene blue (Chem.), an artificial dyestuff consisting
      of a complex sulphur derivative of diphenyl amine; --
      called also pure blue.
      [1913 Webster]

WordNet (r) 3.0 (2006):

methylene
    n 1: the bivalent radical CH2 derived from methane [syn:
         methylene group, methylene radical, methylene]