Other galaxies were called, at first, 'island universes'.
When William Herschel constructed his catalog of deep sky objects in 1786, he used the name spiral nebula for certain objects such as M31. These would later be recognized as immense conglomerations of stars, when the true distance to these objects began to be appreciated, and they would be termed island universes. However, the word Universe was understood to mean the entirety of existence, so this expression fell into disuse and the objects instead became known as galaxies.
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In the 10th century, the Persian astronomer Al-Sufi made the earliest recorded observation of the Andromeda Galaxy, describing it as a "small cloud".[37] Al-Sufi, who published his findings in his Book of Fixed Stars in 964, also identified the Large Magellanic Cloud, which is visible from Yemen, though not from Isfahan; it was not seen by Europeans until Magellan's voyage in the 16th century.[38][39] The Andromeda Galaxy was independently rediscovered by Simon Marius in 1612.[37] These are the only galaxies outside the Milky Way that are easily visible to the unaided eye, so they were the first galaxies to be observed from Earth. In 1750 Thomas Wright, in his An original theory or new hypothesis of the Universe, speculated (correctly) that the Milky Way was a flattened disk of stars, and that some of the nebulae visible in the night sky might be separate Milky Ways.[32][40] In 1755, Immanuel Kant used the term "island Universe" to describe these distant nebulae.
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In 1917, Heber Curtis had observed a nova S Andromedae within the "Great Andromeda Nebula" (as the Andromeda Galaxy, Messier object M31, was known). Searching the photographic record, he found 11 more novae. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within our galaxy. As a result he was able to come up with a distance estimate of 150,000 parsecs. He became a proponent of the so-called "island universes" hypothesis, which holds that spiral nebulae are actually independent galaxies.
https://en.wikipedia.org/wiki/Galaxy
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