The Collaborative International Dictionary of English v.0.48:

Paraxylene \Par`a*xy"lene\, n. (Chem.)
   A hydrocarbon (C6H4(CH3)2) of the aromatic series obtained
   as a colorless liquid by the distillation of camphor with
   zinc chloride. It is one of the three metamers of xylene. Cf.
   Metamer, and Xylene.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Xylene \Xy"lene\, n. [Gr. xy`lon wood.] (Chem.)
   Any of a group of three metameric hydrocarbons of the
   aromatic series, found in coal and wood tar, and so named
   because found in crude wood spirit. They are colorless, oily,
   inflammable liquids, C6H4.(CH3)2, being dimethyl benzenes,
   and are called respectively orthoxylene, metaxylene, and
   paraxylene. Called also xylol.
   [1913 Webster]

   Note: Each of these xylenes is the nucleus and prototype of a
         distinct series of compounds.
         [1913 Webster]