The Collaborative International Dictionary of English v.0.48:

Flipper \Flip"per\, n. [Cf. Flip, Flippant.]
   1. (Zool.) A broad flat limb used for swimming, as those of
      seals, sea turtles, whales, etc.
      [1913 Webster]

   2. (Naut.) The hand. [Slang]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

flippers \flippers\ n. flipper [By analogy with
   flipper[W13(1)], the limb of an aquatic animal used for
   locomotion]
   A type of shoe with a paddle-like front extending well beyond
   the end of the toe, used an aid in swimming (especially
   underwater).

   Note: They are typically of rubber-like material, and are not
         worn when walking on land.

   Syn: fin, fins.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

flipper
    n 1: a shoe for swimming; the paddle-like front is an aid in
         swimming (especially underwater) [syn: flipper, fin]
    2: the flat broad limb of aquatic animals specialized for
       swimming