1. Chem.any of three isomeric acids having the formula C8H6O4, 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.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.