An Etymological Dictionary of Astronomy and Astrophysics

1. 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 neighborhood around its orbit. → dwarf planet.
   
2. → extrasolar planet.

Etymology (EN): From O.E., from O.Fr. planete (Fr. planète), from L.L. planeta (plural form planetae), from Gk. planetes (single form) “wandering,” from (asteres) planetai “wandering (stars),” from planasthai “to wander,” of unknown origin.

Etymology (PE): Sayyâré, from Ar. saiyârat “walker, traveller.”