U-boat

Submarine Sub`ma*rine", n. 1. A submarine boat; a ship that can travel under the surface of the water. Most such ships are ships of war, as part of a navy, but submarines are also used for oceanic research. Also called {sub} and (from the German U-Boot) {U-boat}. esp., (Nav.), a submarine torpedo boat; -- called specif. {submergible submarine} when capable of operating at various depths and of traveling considerable distances under water, and {submersible submarine} when capable of being only partly submerged, i.e., so that the conning tower, etc., is still above water. The latter type and most of the former type are submerged as desired by regulating the amount of water admitted to the ballast tanks and sink on an even keel; some of the former type effect submersion while under way by means of horizontal rudders, in some cases also with admission of water to the ballast tanks. [Webster 1913 Suppl. +PJC]

2. A stowaway on a seagoing vessel. [Colloq.] [PJC]

3. A {submarine sandwich}. [PJC]

{Nuclear submarine} A submarine powered by a nuclear reactor.

{Attack submarine} A submarine designed to attack other ships, including other submarines.