a market-town in the parish of Llangattock, and hundred of Usk, Monmouthshire, stands on the right bank of the river Usk, N. Lat. 51.37., W. Long. 2.56.; 2 miles N.E. of Newport, and 148 miles west from London. It covers an area of 450 acres, partly on the site of the ancient Isca Silurum, the station of the second legion (of which leon is a contraction), and is thought at one time to have been the capital of Wales. It is an old and irregular town, with an ancient parish church (St Cadoc's), and three other places of worship, besides an endowed, a national, and an infant school. The iron and tin-plate works give employment to a large number of the inhabitants, but otherwise there is little trade of any description in the place. In the Caerleon neighbourhood are the remains of the Roman city, with walls, a fortress, and an amphitheatre since called Arthur's Round Table. And a great variety of antiquities, chiefly Roman altars, pavements, tiles, coins, medallions, &c., have been dug up in the vicinity, and are deposited in a museum erected for that purpose in the town. Caerleon was the seat of an ancient archbishopric, afterwards removed to St David's. Pop. (1851) 1281.