Any natural object that revolves around a larger astronomical object, usually a planet. The Moon is the most obvious example.
All the planets in the solar system except Mercury and Venus have natural satellites. More than 60 such objects have so far been discovered. Saturn has the most satellites, with 18 certain and several more yet to be confirmed. The natural satellites vary greatly in size. Some of them measure only several kilometres in diameter, as in the case of the two tiny moons of Mars and the outer satellites of Jupiter. A few are larger than Mercury, as for example Saturn's Titan and Jupiter's Ganymede and Callisto, each of which is more than 5,200 km (3,230 miles) in diameter. The satellites also differ significantly in composition. The Moon, for example, consists almost entirely of rocky material. On the other hand, the composition of Saturn's Enceladus is 50 percent or more ice.
Excerpt from the Encyclopedia Britannica without permission.