Planet (noun): a celestial body that orbits the Sun, has sufficient mass to assume hydrostatic equilibrium and to clear its orbital path of debris, and is not a moon.
Dwarf planet (noun): a celestial body that orbits the Sun, is not a moon, has sufficient mass to assume hydrostatic equilibrium but not to clear its orbital path of debris.
A dwarf planet, thus, has almost exactly the same definition as a planet, except that it does not have enough gravity to clear its orbit. Because gravity is dependent on mass, the essential difference between a planet and a dwarf planet is mass.
"Planet" comes from the Greek planetoi, "wanderers." This is because the stargazing Greeks saw the planets and noted that they did not follow the same path in the sky as the stars; they wandered through the skies. Thus the name meaning "wanderer."
Bibliography
1. Moche, Dinah L. Astronomy: A Self-Teaching Guide. Hoboken, NJ: John Wiley & Sons, Inc., 2009.
2. Wikipedia. "IAU definition of planet." Wikipedia. 16 May 2013. Wikipedia. May 21, 2013. <http://en.wikipedia.org/wiki/IAU_definition_of_planet>
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