A moon, or a natural satellite, is a celestial body that orbits a planet or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify non-artificial satellites of planets, dwarf planets, and minor planets.
Large gas giants have extensive systems of moons.
Limits on the size of a moon
There is no established lower limit on what should be considered a moon: Every satellite with an identified orbit, some as small as a kilometer across, has been identified as a moon, though clumps a tenth that size within planetary rings (which may not be solid bodies) have been called moonlets. (Small asteroid moons, such as Dactyl, some up to tens of kilometers across, have also been called moonlets.) The upper limit is also vague: When the masses of two orbiting bodies are similar enough that one cannot be said to orbit the other, they are described as a dual body rather than primary and satellite.
The natural satellites orbiting relatively close to the planet on prograde orbits (regular satellites) are generally believed to have been formed out of the same collapsing region of the protoplanetary disk that gave rise to its primary. In contrast, irregular satellites (generally orbiting on distant, inclined, eccentric and/or retrograde orbits) are thought to be captured asteroids possibly further fragmented by collisions. Some exceptional large bodies are believed to have originated by the collision of two large proto-planetary objects. The material that would have been placed in orbit around the central body is predicted to have reaccreted to form one or more orbiting moons. As opposed to planetary-sized bodies, asteroid moons are thought to commonly form by this process.
Many natural satellites of standard planets are tidally locked to their primaries, meaning that one side of the moon is always turned toward the planet. In contrast, the outer moons of gas giants (irregular satellites) are too far away to become 'locked'.
Satellites of satellites
"Moons of moons" (natural satellites that orbit the natural satellite of another body) are rare, and such objects may not be stable in the long term. In most cases, the tidal effects of their primaries make such a system unstable; the gravity from other nearby objects (most notably the primary) would perturb the orbit of the moon's moon until it broke away or impacted its primary.
Some moons have been observed to possess narrow stable ring systems.
Some moons are known to have small companions at their P4 and P5 gravitational equilibrium points, which are about sixty degrees ahead of and behind the body in its orbit. These companions are called Trojan moons.
Some asteroids also have moons. See asteroid moon for further information.