What Exactly is a Galaxy?

sciencehabit writes: Surprising as it may sound, astronomers don't have an answer to this fundamental question. There's no agreement on when a collection of stars stops being a cluster and starts being something more. Now, in an echo of the recent wrangling over Pluto's status as a planet, a pair of astrophysicists from Australia and Germany want to start a debate on the issue—and they have even set up a Web site for people to cast their votes.

Fundamental?

[osu-neko]

I usually reserve "fundamental question" for basic questions about reality, not questions of nomenclature. What is or isn't a planet or a galaxy is a debate about how we should use language, not a debate about Nature or "facts" of any kind. Clear definitions are important for communication among professionals, but it's not what I'd call a "fundamental question".