Which of the following definitions do you like?

Definition 1

    A planet is a celestial body that
  1. has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
  2. is in orbit around a star, and is neither a star nor a satellite of a planet.
This means there would be 12 planets in our solar system: Mercury, Venus, Earth, Mars, Ceres (currently an asteroid), Jupiter, Saturn, Uranus, Neptune, Pluto, Charon and 2003 UB313 (in the Kuiper Belt of debris in the outer parts of our solar system). Those beyond Neptune's orbit would be called Plutons.

See http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_release.html

Definition 2

  1. A classical "planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
  2. A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape2, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
  3. All other objects3, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".
This means that there would be 8 planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

See http://www.iau2006.org/mirror/www.iau.org/iau0602/index.html

What Astronomers and Planetary Scientists Think

Check out this statement by the Division of Planetary Scientists.