Phasis, the ancient name of the Rion or Faz, a river of western Asia, in Transcaucasia, which rises at the foot of Mt. Pasmta in the western division of the Caucasus, flows S. W. and W. for about 50 m., S. for more than 40 m., and W. for about 60 m., and enters the Black sea about lat. 42° 10' N, Ion. 41° 40' E. It receives in succession the Kvirila, the Tzkhate, and the Tekhur, besides several smaller streams. It is navigable in its lower course, and forms an important transport facility for Kutais and other inland towns. The railway from Tiflis to the sea follows the lower (westerly) portion of its course. Its width at Kutais varies from about 125 (in very hot summers) to nearly 1,000 ft. The Phasis was an important river in the ancient world, being considered by the oldest geographers the boundary between Europe and Asia, and by later classical writers between Asia Minor and Colchis, of which latter country it was the principal stream. The three affluents named above seem to be respectively the Rhium, the Hippius, and (probably) the Glaucus of the ancients, and the modern Kutais was their Cytaea or Cuta-tisium. The legend of the Argonauts represents them as landing at the mouth of the Phasis. - The town of Phasis was a well known ancient trading port, founded and strongly fortified by the Milesians a little south of the river's mouth, near the modern Poti. - Our word pheasant (the avis Phasiana of the Romans) is derived from the supposed origin of that bird in the neighborhood of the Phasis.