Adour (anc. Aternvs), a river in the S. W. of France, about 180 m. in length, 70 of which are navigable. Its course is nearly semicircular. It rises in the Pyrenees, flows through the departments of Hautes-Pyrenees, Gers, and Landes, and empties into the bay of Biscay, a little below Bayonne. Though many streams unite with it, its volume of water is small, except during the melting of the snows in the Pyrenees, when it often inundates the surrounding country.