The Collaborative International Dictionary of English v.0.48:

Pluto \Plu"to\ (pl[=u]"t[-o]), pr. n. [L., fr. Gr. Plou`twn.]
   1. (Class. Myth.) The son of Saturn and Rhea, brother of
      Jupiter and Neptune; the dark and gloomy god of the Lower
      World.
      [1913 Webster]

   2. The ninth planet of the Solar System, the smallest (5700
      km radius) and most distant from the sun. The suggestion
      has been made that it more closely resembles a large close
      comet than a planet. Its orbit has an eccentricity of
      0.248, larger than that of any other planet; it varies
      from 4.44 to 7.37 billion km distance from the sun.
      [PJC]

            Pluto is an oddball among its eight sister planets.
            It's the smallest in both size and mass, and has the
            most elliptical orbit. It moves in a plane tilted
            markedly away from the other planets' orbits.
            Moreover, Pluto is the only planet made almost
            entirely of ice.                      --Ron Cohen
                                                  (Science News,
                                                  Feb. 27, 1999,
                                                  p. 139)

   Pluto monkey (Zool.), a long-tailed African monkey
      (Cercopithecus pluto), having side whiskers. The general
      color is black, more or less grizzled; the frontal band is
      white.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Pluto
    n 1: a cartoon character created by Walt Disney
    2: (Greek mythology) the god of the underworld in ancient
       mythology; brother of Zeus and husband of Persephone [syn:
       Pluto, Hades, Aides, Aidoneus]
    3: a small planet and the farthest known planet from the sun; it
       has the most elliptical orbit of all the planets; "Pluto was
       discovered by Clyde Tombaugh in 1930"