Definition: planet: A planet is a spherical ball of rock and/or gas that orbits a star. The Earth is a planet. Our solar system has eight planets. These planets are, in order of increasing average distance from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The only object visited by humans other than the planet Earth is the Moon, which is not a planet. It is a satellite.
The International Astronomy Union has an official definition for a planet. It states, "A 'planet' is defined as 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."