If we just go by the dictionary definition.
   " galaxy
ˈɡaləksi/

noun: galaxy; plural noun: galaxies

    a system of millions or billions of stars, together with gas and dust, held together by gravitational attraction.
    synonyms:        star system, solar system, constellation, cluster,"
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What has to be present for something to be called a galaxy ? would be a simplistic way of answering the OP.