The word "planet" means a celestial body that orbits a star, such as the Sun, and is large enough for its own gravity to make it nearly round and to clear its orbit of other debris. The term comes from the ancient Greek word "planētēs," meaning "wanderer," because planets appear to move across the sky relative to the fixed stars. Modern definitions, such as the one by the International Astronomical Union (IAU) in 2006, specify that a planet must orbit the Sun, have sufficient mass to be nearly round, and have cleared its orbit.