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]