VESTERAS, or WESTERAS  , a town and bishop's see of Sweden, capital of the district (Jan) of Vestmanland, on a northern bay of Lake Malar, 6o m . N.W. by W. of Stockholm by rail . Pop . (1900) 11,999 . It is a considerable industrial centre and an important lake port . Its Gothic cathedral, rebuilt by Birger Jarl on an earlier site, and consecrated in 1271, was restored in 1850-186o, and again in 1896-1898 . The episcopal library contains the valuable collection of books which Oxenstjerna, the chancellor of Gustavus Adolphus, brought away from Mainz near the end of the Thirty Years' War . A castle commands the town from an eminence; it was captured by Gustavus Vasa and rebuilt by him, and again in the 17th century, and remains the seat of the provincial government . Here Eric XIV., whose tomb is in the cathedral., was confined (1573-1575) . Several national diets were held in this town, the most notable being those of 1527, when Gustavus Vasa formally introduced the Reformation into Sweden, and 1544, when he had the Swedish throne declared hereditary in his family . The original name of the town was Vestra Aros (" western mouth "), in distinction from Ostra Aros, the former name of Upsala .