German U-boat

U-boat is the anglicised version of the German word U-Boot [ˈuːboːt] ( listen), a shortening of Unterseeboot, literally "undersea boat".^[1] While the German term refers to any submarine, the English one (in common with several other languages) refers specifically to military submarines operated by Germany, particularly in the First and Second World Wars. Although at times they were efficient fleet weapons against enemy naval warships, they were most effectively used in an economic warfare role (commerce raiding), 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 Canada, the British Empire, and the United States to the United Kingdom and (during the Second World War) to the Soviet Union and the Allied territories in the Mediterranean.

Austro-Hungarian navy submarines were also known as U-boats