Island in Denmark. Located between the Kattegat strait and the Baltic Sea, Sjælland is the largest (2,715 sq mi [7,031 sq km]) and most populous island (pop., 1990 est.: 1,972,711) of Denmark. Copenhagen is its major city. Two fjords break the irregular coastline of the island, and fine beaches line its northern shore. It has many Stone Age and Viking relics, including the Viking fortress of Trælleborg (c. 1000), as well as medieval churches, castles, and manor houses. Grain and livestock farming, fishing, and tourism are major industries.