Thun (tōn) [key], city (1990 est. pop. 37,700), Bern canton, central Switzerland, on the Aare River, where it flows out of the Lake of Thun. Machinery, watches, pottery, and cheese are made there. Of interest are two castles, the 16th-century town hall, and the historical museum. The Lake of Thun, c.20 sq mi (52 sq km), is picturesquely situated at the foot of the Bernese Alps.