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The Collaborative International Dictionary of English v.0.48 (gcide)

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]