A pluton is an orbital body that is distinguished from a traditional planet by the length of its orbit, the aspect of that orbit and the shape of the orbit, which is far from circular.
The International Astronomical Union (IAU) completed a new draft resolution in 2006 that refined the definition for planet to be the following:
A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet."
In the draft resolution, the IAU added three more planets to the solar system in which Earth resides, bringing the total to twelve. These new planets are all plutons. Plutons all reside in orbits that take longer than 200 years to complete, placing them beyond Neptune in proximity to the center of the solar system. They are also distinguished by a high degree of orbital eccentricity.