IAU Resolution: Definition of a Planet
Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflects our current understanding. This applies, in particular to the designation 'planets'. The word 'planet' originally described 'wanderers' that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information. (Here we are not concerned with the upper boundary between 'planet' and 'star', nor the distinction between stars and brown dwarfs).
RESOLUTION 5 (draft c)
The IAU therefore resolves that planets and other bodies in the planetary systems be defined in the following way:
(1) A planet is a celestial body that (a) has sufficient mass for its self- gravity to overcome rigid body forces so that is assumes a hydrostatic equilibrium (nearly round) shape, (b) is the dominant object in its local population zone and (c) is in orbit around the Sun.
(2) A dwarf-planet is a celestial body that (a) has sufficient mass for its self- gravity to overcome rigid body forces so that is assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around the Sun.
(3) All other objects orbiting the Sun, including most of the Solar System asteroids, near-Earth objects (NEOs), Mars-, Jupiter-, Neptune-Trojan asteroids, most Centaurs, most Trans-Neptunian Objects (TNOs), and comets, shall be referred to collectively as "Small Solar System Bodies". In the new nomenclature the term "minor planet" is not used.
 An AIU process will be established to assign borderline objects into either dwarf-planet and other categories.
RESOLUTION 6 (draft c)
The IAU further resolves:
Pluto is a dwarf planet by the above definition, as are one or more recently discovered large trans-Neptunian objects. Dwarf-planets that have orbital periods in excess of 200 years are designated this category of planetary objects, of which Pluto is the prototype, as a new class that we call "plutonids".