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A dwarf planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting the sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite. MOre explicitly, it has to have sufficient mass to overcome its compressive strength and achieve hydrostatic equilibrium.

The term dwarf planet was adopted in 2006 as part of a three-way categorization of bodies orbiting the sun, brought about by an increase in discoveries of trans-Neptunian objects (TNOs) that rivaled Pluto in size, and finally precipitated by the discovery of an even more massive object, Eris. This classification states that bodies large enough to have cleared the neighborhood of their orbit are defined as planets, while those that are not massive enought o be rounded by their own gravity are defined as small solar system bodies. Dwarf planets come in between. The definition officially adopted in 2006 by the IAU has been both praised and criticized, and has been disputed by scientists such as Alan Stern.

The IAU currently recognizes five dwarf planets: Ceres, Pluto, Haumea, Makemake, and Eris. However, only two of these bodies, Ceres and Pluto, have been observed in enough detail to demonstrate that they fit the definition. Eris has been accepted as a dwarf planet because it is more massive than Pluto. The IAU subsequently decided that unnamed TNOs with an absolute magnitude brighter than +1 (and hence a mathematically delimited minimum diameter of 838 km) are to be named under the assumption that they are dwarf planets. The only two such objects known at the time, Haumea and Makemake, went through this naming procedure and were declared to be dwarf planets.