ALSEN, an island in the Baltic, situated off the coast of Schleswig, in the Little Belt. It formerly belonged to Denmark, but, as a result of the Danish war of 1864, was incorporated with Germany. Its area is 105 square miles; the length nearly 20, and the breadth from 3 to 12 miles. The island is fertile, richly wooded, and yields grain and fruit. Sonderburg, the capital, a town of 5475 inhabitants, with a good harbour and a considerable trade, is situated on the narrow channel that separates Alsen from the mainland. Population, 22,500.