A planet is a celestial body that:
#1) is in orbit around the Sun,
#2) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and #3) has cleared the neighborhood around its orbit.
Under this definition, Pluto is NOT a planet, but has been deemed a dwarf planet because it has not yet cleared its orbit. This definition is under discussion, particularly by members of the planetary science community, and it may yet be further refined.
