dwarf planet

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A celestial body that orbits the sun and is massive enough to assume a nearly spherical shape, but that does not clear other bodies from the neighborhood around its orbit and is not a satellite of a planet.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun astronomy Any astronomical body which has achieved hydrostatic equilibrium (which is the shape a body of water would take, as opposed to a small Solar System body), orbits the Sun (as opposed to a moon), and has not "cleared its neighbourhood" of competing bodies (as opposed to a planet).