subsatellite

A subsatellite (or submoon or moonmoon or grandmoon, as per "grandchild") is a satellite of a moon, i.e., a body orbiting a body that orbits a planet. The term may be used for artificial satellites as well as natural ones and space probes have orbited the Moon, some still doing so. However, no natural subsatellites are known within the solar system, though they remain a possibility. Extra-solar planets showing signs of substantial moons (e.g., through transit timing variations) are also candidates for having them.

Note that subsatellite is also an aerospace term for a component of an artificial satellite which is released into space so as to separately orbit the Earth (or whatever body).