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

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "flipper":
   Australian crawl, act drop, aquaplaning, aquatics, asbestos,
   asbestos board, backdrop, backstroke, balneation, bathe, bathing,
   batten, border, breaststroke, butterfly, cloth, coulisse,
   counterweight, crawl, curtain, curtain board, cyclorama, decor,
   diving, dog paddle, drop, drop curtain, fin, fire curtain,
   fishtail, flapper, flat, floating, hanging, natation, rag, scene,
   scenery, screen, side scene, sidestroke, stage screw, surfboarding,
   surfing, swim, swimming, tab, tableau, teaser, tormentor,
   transformation, transformation scene, treading water, wading,
   waterskiing, wing, wingcut, woodcut