What is the detailed, universal classification that the term 'galaxy' was adopted to describe?

Any massive, gravitationally bound system of stars, stellar remnants, gas, dust, and dark matter.

While the term 'galaxy' is derived from the Greek word for milk (*gala*) due to its application to our local system, its modern scientific utility extends far beyond that initial description. Once astronomers understood that structures like the Andromeda Nebula (M31) were independent systems, the word 'galaxy' was generalized. It now serves as the formal, comprehensive astrophysical classification for any colossal structure held together by gravity, encompassing all its constituent parts—which include not only observable stars and gas clouds but also crucial components like stellar remnants and necessary invisible elements such as dark matter.