The Ancient Greek word gala, meaning “milk,” is the root behind the word galaxy. Greek writers called the Milky Way galaxias, a term formed from gala and related milk-root vocabulary.
That name arose from the Milky Way’s appearance as a pale, milk-like band across the night sky. In Greek tradition, galaxias was short for a phrase meaning “milky circle,” reflecting the way the luminous band appears to wrap around the celestial sphere.
The word galaxy eventually broadened from the name of our own Milky Way to a general term for a vast system of stars, gas, dust and dark matter. Thus, a word describing a visual impression became the standard label for objects far beyond our own stellar system.
The mythological explanation is also connected to the same word family: one Greek story says the band formed from divine milk associated with Hera and the infant Heracles. The scientific explanation, however, is that we are viewing the crowded disk of our galaxy from inside it.