In 2006, the IAU decided to create a new classification category for solar system objects called "dwarf planets" that is distinct from "planets". Pluto was reclassified as a dwarf planet. At present there are only four other members of this category, Ceres, Eris, Makemake and Haumea but it is likely that there will be many more in the future as the Kuiper Belt is more fully explored.
The actual definition of "dwarf planet" is kind of technical: a celestial body that
In short, something that looks like a planet but isn't a planet.
Dwarf planets so far:
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Bill Arnett; last updated: 2009 Jan 1