Both etymologically and scientifically speaking, there used to be a lot less galaxies than there are today. Though the scientific explanation is fun too, let's delve into the word origin: it can be traced back to the Proto-Indo-European word glakt, which is reconstructed meaning "milk". This may sound familiar, since its root went into Latin as lac-, which is present in lactose and lactate and a bunch of milk-related terms. But how does this connect to galaxy? Well, glakt went into Greek as galaktos, and then just gala, both still meaning "milk". However, this changed, since the Greeks were avid astronomers, and liked to name things up there in the heavens. Thus, the "Milky Way" became known as the galaxias kyklos, or "milky circle" (Milky Way itself is a translation of a Latin term deriving from this). Eventually people dropped the unnecessary "circle" part, but this still referred exclusively to our galaxy (because for all we knew back then, it was the only one). This then passed through Latin and French to enter English, and later became an official astronomer's term for a large collection of solar systems bound together by gravity. The "Milky Way" definition is only remembered by the heavens.
Adam Aleksic, a leading contender for valedictorian of his high school, is a 208-month-old boy with disturbing interests in etymology, vexillology, geography, and law. Adam would like to one day visit Tajikistan and just won an essay contest on linguistics!
The Etymology Nerd