Galaxies are found in groups called galaxy clusters which are also held together by gravitation. Our own Milky Way is the second largest galaxy in our Local Group (the largest is Andromeda). Many smaller galaxies and star clusters are also held in the Local Group, outside the two main galaxies. They are all in orbits round one of the centres of gravity. That means most of them move round either Andromeda or the Milky Way. so it seems natural for astronomers to use the term 'satellite' for these as well.
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