U-Boat.

"U-boat is the anglicized version of the German word U-Boot (help•info), itself an abbreviation of Unterseeboot (undersea boat), and refers to military submarines operated by Germany, particularly in World War I and World War II. Although in theory, U-boats could have been useful fleet weapons against enemy naval warships, in practice they were most effectively used in an economic-warfare role, enforcing a naval blockade against enemy shipping. The primary targets of the U-boat campaigns in both wars were the merchant convoys bringing supplies from the British Empire and the United States to the island of Great Britain. Austrian submarines of World War I were also known as ""U-Boats"". The distinction between U-boat and submarine is common in English-language usage (where U-boat refers exclusively to the German vessels of the World Wars) but is unknown in German, in which the term U-Boat refers to any submarine. "