n. An aromatic hydrocarbon based on benzene with two methyl substituents bonded to adjacent carbon atoms in the aromatic ring (the ortho configuration).
the GNU version of the Collaborative International Dictionary of English
n. That variety of xylene (C6H4(CH3)2) in which the two methyl groups are in the ortho position; a colorless, liquid, combustible hydrocarbon resembling benzene.
Word Usage
"If you photooxidize orthoxylene, with it's two adjacent methyl groups, when the ring opens, a certain percentage of time you get biacetyl."