Pronunciation: (sim'plon; Fr. san-plôn'),[key] —n. 1. a mountain pass in S Switzerland, in the Lepontine Alps: crossed by a carriage road constructed 1800–06 on Napoleon's orders. 6592 ft. (2010 m) high.
2. a tunnel between Switzerland and Italy, NE of the Simplon Pass: longest tunnel in the world. 12¼ mi. (20 km) long.