The Collaborative International Dictionary of English v.0.48:

Fin \Fin\, n.[OE. finne, fin, AS. finn; akin to D. vin, G. &
   Dan. finne, Sw. fena, L. pinna, penna, a wing, feather. Cf.
   pen a feather.]
   1. (Zool.) An organ of a fish, consisting of a membrane
      supported by rays, or little bony or cartilaginous
      ossicles, and serving to balance and propel it in the
      water.
      [1913 Webster]

   Note: Fishes move through the water chiefly by means of the
         caudal fin or tail, the principal office of the other
         fins being to balance or direct the body, though they
         are also, to a certain extent, employed in producing
         motion.
         [1913 Webster]

   2. (Zool.) A membranous, finlike, swimming organ, as in
      pteropod and heteropod mollusks.
      [1913 Webster]

   3. A finlike organ or attachment; a part of an object or
      product which protrudes like a fin, as:
      (a) The hand. [Slang]
      (b) (Com.) A blade of whalebone. [Eng.] --McElrath.
      (c) (Mech.) A mark or ridge left on a casting at the
          junction of the parts of a mold.
      (d) (Mech.) The thin sheet of metal squeezed out between
          the collars of the rolls in the process of rolling.
          --Raymond.
      (e) (Mech.) A feather; a spline.
          [1913 Webster]

   4. A finlike appendage, as to submarine boats.
      [1913 Webster]

   5. (A["e]ronautics) A fixed stabilizing surface, usually
      vertical, similar in purpose to a bilge keel on a ship.
      [Webster 1913 Suppl.]

   Apidose fin. (Zool.) See under Adipose, a.

   Fin ray (Anat.), one of the hornlike, cartilaginous, or
      bony, dermal rods which form the skeleton of the fins of
      fishes.

   Fin whale (Zool.), a finback.

   Paired fins (Zool.), the pectoral and ventral fins,
      corresponding to the fore and hind legs of the higher
      animals.

   Unpaired fins, or Median fins (Zool.), the dorsal,
      caudal, and anal fins.
      [1913 Webster]