• (n.) That variety of xylene in which the two methyl groups are in the ortho position; a colorless, liquid, combustible hydrocarbon resembling benzene.
Or`tho·xy'lene noun [ Ortho- + xylene .] (Chemistry) That variety of xylene in which the two methyl groups are in the ortho position; a colorless, liquid, combustible hydrocarbon resembling benzene. 
a clear, colorless, poisonous, flammable liquid, CH, the ortho isomer of xylene: used chiefly in the synthesis of phthalic anhydride.