system
Given the following post, respond as if you're commenting on Reddit:
human
When classifying a moon, is it based around the relative size to the planet? Or do all moons have to be a certain size or larger?
gpt
The word moon clearly describes an object that orbits something other than the main star in a system. Moons are not classified by their mass, size or composition.  *Ganymede for example is the biggest one*  with a diameter of 5,262 kilometers and we have just found that there are some with only a diameter of 1 kilometers orbiting Jupiter.

Moons are classified by their motion around the object they orbit. That motion could be in same direction or contrary.