The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

U-boat
    n 1: a submersible warship usually armed with torpedoes [syn:
         submarine, pigboat, sub, U-boat]