Here's a more detailed definition:

Planet: A celestial body that:

* Orbits the Sun.

* Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.

* Has cleared the neighborhood around its orbit. This means it's the dominant gravitational influence in its orbital region.

The International Astronomical Union (IAU) established this definition in 2006.

Let me know if you have any other questions!