The Collaborative International Dictionary of English v.0.48:

Methyl \Meth"yl\, n. [See Methylene.] (Chem.)
   A univalent hydrocarbon radical, CH3-, not existing alone
   but regarded as an essential residue of methane, and
   appearing as a component part of many derivatives; as, methyl
   alcohol, methyl ether, methyl amine, etc. [Formerly written
   also methule, methyle, etc.]
   [1913 Webster]

   Methyl alcohol (Chem.), a light, volatile, inflammable
      liquid, CH3.OH, obtained by the distillation of wood,
      and hence called wood alcohol or wood spirit;
      tecnically referred to as methanol; -- called also
      methol, carbinol, etc.

   Methyl amine (Chem.), a colorless, inflammable, alkaline
      gas, CH3.NH2, having an ammoniacal, fishy odor. It is
      produced artificially, and also occurs naturally in
      herring brine and other fishy products. It is regarded as
      ammonia in which a third of its hydrogen is replaced by
      methyl, and is a type of the class of substituted
      ammonias.

   Methyl ether (Chem.), a light, volatile ether CH3.O.CH3,
      obtained by the etherification of methyl alcohol; --
      called also methyl oxide or dimethyl ether.

   Methyl green. (Chem.) See under Green, n.

   Methyl orange. (Chem.) See Helianthin.

   Methyl violet (Chem.), an artificial dye, consisting of
      certain methyl halogen derivatives of rosaniline.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Helianthin \He`li*an"thin\, n. [Prob. fr. L. helianthes, or NL.
   helianthus, sunflower, in allusion to its color.] (Chem.)
   An artificial, orange dyestuff, analogous to tropaolin, and
   like it used as an indicator in alkalimetry; -- called also
   methyl orange.
   [1913 Webster]

WordNet (r) 3.0 (2006):

methyl orange
    n 1: an azo dye used as an acid-base indicator; used for
         titrations involving weak bases