Gustavus n 1: king of Sweden whose victories in battle made Sweden a European power; his domestic reforms made Sweden a modern state; in 1630 he intervened on the Protestant side of the Thirty Years' War and was killed in the battle of Lutzen (1594-1632) [syn: {Gustavus II}, {Gustavus Adolphus}] 2: king of Sweden who established Lutheranism as the state religion (1496-1560) [syn: {Gustavus I}]