Definitions:

1. A German submarine, particularly those used in naval warfare during the 20th century, most notably in efforts to disrupt enemy shipping in both World Wars. NOUN
   • During World War I, the U-boat campaign severly impacted British supply lines.
2. A submarine warship usually armed with torpedoes. NOUN
   • The U-boat menace posed a significant challenge to Allied naval operations.
3. Shortened form for the German word "Unterseeboot", meaning "Undersea boat", and refers to a military submarine used by Germany, especially during World War I and World War II. NOUN
   • Allied convoys were constantly under threat from German U-boats in the Atlantic.

Synonyms:

• nuclear submarine
• sub
• submarine
• submersible