The Collaborative International Dictionary of English v.0.48:

Duck \Duck\, n. [OE. duke, doke. See Duck, v. t. ]
   1. (Zool.) Any bird of the subfamily Anatin[ae], family
      Anatid[ae].
      [1913 Webster]

   Note: The genera and species are numerous. They are divided
         into river ducks and sea ducks. Among the former
         are the common domestic duck (Anas boschas); the wood
         duck (Aix sponsa); the beautiful mandarin duck of
         China (Dendronessa galeriliculata); the Muscovy duck,
         originally of South America (Cairina moschata). Among
         the sea ducks are the eider, canvasback, scoter, etc.
         [1913 Webster]

   2. A sudden inclination of the bead or dropping of the
      person, resembling the motion of a duck in water.
      [1913 Webster]

            Here be, without duck or nod,
            Other trippings to be trod.           --Milton.
      [1913 Webster]

   Bombay duck (Zool.), a fish. See Bummalo.

   Buffel duck, Spirit duck. See Buffel duck.

   Duck ant (Zool.), a species of white ant in Jamaica which
      builds large nests in trees.

   Duck barnacle. (Zool.) See Goose barnacle.

   Duck hawk. (Zool.)
      (a) In the United States: The peregrine falcon.
      (b) In England: The marsh harrier or moor buzzard.

   Duck mole (Zool.), a small aquatic mammal of Australia,
      having webbed feet and a bill resembling that of a duck
      (Ornithorhynchus anatinus). It belongs the subclass
      Monotremata and is remarkable for laying eggs like a bird
      or reptile; -- called also duckbill, platypus,
      mallangong, mullingong, tambreet, and water mole.


   To make ducks and drakes, to throw a flat stone obliquely,
      so as to make it rebound repeatedly from the surface of
      the water, raising a succession of jets; hence:

   To play at ducks and drakes, with property, to throw it
      away heedlessly or squander it foolishly and unprofitably.


   Lame duck. See under Lame.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Ornithorhynchus anatinus
    n 1: small densely furred aquatic monotreme of Australia and
         Tasmania having a broad bill and tail and webbed feet; only
         species in the family Ornithorhynchidae [syn: platypus,
         duckbill, duckbilled platypus, duck-billed platypus,
         Ornithorhynchus anatinus]