Wikipedia: A planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.

Scholastic Children’s Dictionary: A planet is one of the nine large heavenly bodies circling the sun.

Houghton Mifflin Children’s Dictionary: A planet is a heavenly body that moves in an orbit around a star such as the sun.