Source : Webster's Revised Unabridged Dictionary (1913)

Keel \Keel\, n. [Cf. AS. ce['o]l ship; akin to D. & G. kiel
   keel, OHG. chiol ship, Icel. kj[=o]ll, and perh. to Gr.
   gay^los a round-built Ph[oe]nician merchant vessel, gaylo`s
   bucket; cf. Skr. g[=o]la ball, round water vessel. But the
   meaning of the English word seems to come from Icel. kj["o]lr
   keel, akin to Sw. k["o]l, Dan. kj["o]l.]
   1. (Shipbuilding) A longitudinal timber, or series of timbers
      scarfed together, extending from stem to stern along the
      bottom of a vessel. It is the principal timber of the
      vessel, and, by means of the ribs attached on each side,
      supports the vessel's frame. In an iron vessel, a
      combination of plates supplies the place of the keel of a
      wooden ship. See Illust. of {Keelson}.

   2. Fig.: The whole ship.

   3. A barge or lighter, used on the Type for carrying coal
      from Newcastle; also, a barge load of coal, twenty-one
      tons, four cwt. [Eng.]

   4. (Bot.) The two lowest petals of the corolla of a
      papilionaceous flower, united and inclosing the stamens
      and pistil; a carina. See {Carina}.

   5. (Nat. Hist.) A projecting ridge along the middle of a flat
      or curved surface.

   {Bilge keel} (Naut.), a keel peculiar to ironclad vessels,
      extending only a portion of the length of the vessel under
      the bilges. --Ham. Nav. Encyc.

   {False keel}. See under {False}.

   {Keel boat}.
      (a) A covered freight boat, with a keel, but no sails,
          used on Western rivers. [U. S.]
      (b) A low, flat-bottomed freight boat. See {Keel}, n., 3.
          

   {Keel piece}, one of the timbers or sections of which a keel
      is composed.

   {On even keel}, in a level or horizontal position, so that
      the draught of water at the stern and the bow is the same.
      --Ham. Nav. Encyc.

Source : WordNet®

bilge keel
     n : either of two lengthwise fins attached along the outside of
         a ship's bilge; reduces rolling