Definitions
1.NounUsed other than figuratively or idiomatically: see tertiary, planet. (the third planet; usually counting outwards from the central sun)
2.NounA planet of the third-degree/third-order. The planet of a planet (secondary planet) of a planet (primary planet). An object (moonmoon) which orbits an object (moon) which orbits an object (planet) which orbits a star.
3.NounAn aspect in astrology, a calculated node in a reading, that is not a real planet.