Today it means: M31-V1: A star, a Cepheid variable in Andromeda Galaxy, Local Group, Local Supercluster, Laniakea, located in the Andromeda constellation; the first variable star discovered outside the Milky Way Galaxy, and first star identified whose distance was unequivocably beyond the confines of the Milky Way. Full definition →
From Hubble (“surname”) + variable (“variable star”). From having the variable nature of this star being discovered by Edwin Hubble. From this being a very important star in the history of astronomy, and the discovery made using it by Hubble.
Etymology from Wiktionary, CC BY-SA 4.0.