1. Aromatic acid with a carboxyl group in the ortho position (1,2 position). 2. Organic acid with one added molecule of water in chemical combination; for example, HC(OH)₃, orthoformic acid, in contrast to HCOOH, formic acid; H₃PO₄(P₂O₅_3H₂O), orthophosphoric acid, in contrast to the less hydrated form, metaphosphoric acid, HPO₃(P₂O₅_H₂O).