A Quick and Simple Dictionary

planet

1. Noun.  (obsolete, except) Each of the seven major bodies which move relative to the fixed stars in the night sky—the Moon, Mercury, Venus, the Sun, Mars, Jupiter and Saturn.
2. Noun.  A large body which directly orbits a star (or star cluster) but which has not attained nuclear fusion. (The Galilean moons, the first fifteen or so asteroids, and Pluto were included as planets of the Sun in the 17th, 19th, and 20th centuries, respectively.).
3. Noun.  (officially, since 2006) A body which orbits the Sun directly and is massive enough to be in hydrostatic equilibrium (effectively meaning a spheroid) and to have "cleared its orbit" (is the clearly dominant body in its neighborhood); specifically, the eight major bodies of Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
4. Noun.  (minority usage since 2006) A body which orbits the Sun directly and is massive enough to be in hydrostatic equilibrium: the eight major planets, as above, as well as the many dwarf planets, including Pluto.
5. Noun.  (minority usage) A planetary-mass object (planemo): the major planets, dwarf planets, and satellite planets (moons in hydrostatic equilibrium).