U-boat is the anglicized
version of the German word U-Boot, 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-Boot refers to any submarine.