A category of planet introduced in 2006 by the International Astronomical Union. As defined by the IAU, a dwarf planet has mass sufficient for its own gravity to have rendered it spherical or near-spherical, yet too small to have removed material around its orbit by accretion or gravitational disturbance. The first five recognized dwarf planets are Ceres, Eris, Pluto, Haumea, and Makemake. Dozens more may exist beyond Neptune.

http://web.gps.caltech.edu/~mbrown/dps.html Information page about dwarf planets by one of their discoverers.