orthoxylene \or`thoxy"lene\, n. ortho- + xylene. chem.
   that variety of xylene c6h4ch32 in which the two methyl
   groups are in the ortho position; a colorless, liquid,
   combustible hydrocarbon resembling benzene.
   1913 webster
see also:
c6h4ch32 

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.ch32, 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
see also:
c6h4.ch32 orthoxylene metaxylene paraxylene xylol