The Collaborative International Dictionary of English v.0.48:

Nautilus \Nau"ti*lus\, n.; pl. E. Nautiluses, L. Nautili.
   [L., fr. Gr. nayti`los a seaman, sailor, a kind of shellfish
   which was supposed to be furnished with a membrane which
   served as a sail; fr. nay^s ship. See Nave of a church.]
   1. (Zool.) The only existing genus of tetrabranchiate
      cephalopods. About four species are found living in the
      tropical Pacific, but many other species are found fossil.
      The shell is spiral, symmetrical, and chambered, or
      divided into several cavities by simple curved partitions,
      which are traversed and connected together by a continuous
      and nearly central tube or siphuncle. See
      Tetrabranchiata.
      [1913 Webster]

   Note: The head of the animal bears numerous simple tapered
         arms, or tentacles, arranged in groups, but not
         furnished with suckers. The siphon, unlike, that of
         ordinary cephalopods, is not a closed tube, and is not
         used as a locomotive organ, but merely serves to
         conduct water to and from the gill cavity, which
         contains two pairs of gills. The animal occupies only
         the outer chamber of the shell; the others are filled
         with gas. It creeps over the bottom of the sea, not
         coming to the surface to swim or sail, as was formerly
         imagined.
         [1913 Webster]

   2. The argonaut; -- also called paper nautilus. See
      Argonauta, and Paper nautilus, under Paper.
      [1913 Webster]

   3. A variety of diving bell, the lateral as well as vertical
      motions of which are controlled, by the occupants.
      [1913 Webster]

WordNet (r) 3.0 (2006):

nautilus
    n 1: a submarine that is propelled by nuclear power [syn:
         nautilus, nuclear submarine, nuclear-powered
         submarine]
    2: cephalopod mollusk of warm seas whose females have delicate
       papery spiral shells [syn: paper nautilus, nautilus,
       Argonaut, Argonauta argo]
    3: cephalopod of the Indian and Pacific oceans having a spiral
       shell with pale pearly partitions [syn: chambered nautilus,
       pearly nautilus, nautilus]