1. Chem. any of three isomeric acids having the formula C 8 H 6 O 4 , esp. the ortho isomer (orthophthalic acid) , a colorless, crystalline, slightly water-soluble solid used chiefly in the manufacture of dyes, medicine, and perfume.
2. See isophthalic acid .
3. See terephthalic acid .
[ 1855-60 ]