According to our local moris side (northwest) a musician is anybody who is not taking part in the dance being done, but is capable of lifting a large stick and beating the hell out of a bass drum. This also applies to injured dancers and hangers on.

A true musician is one who feels the music from insude and does their best to produce it again from whichever is their preferred instrument.
As for me - I once got a school report which stated that "this pupil is musically dead from the the neck up" which only served to turn me into a banjo freak. I suppose being realistic about this thread, then a musician is probably someone who can justify putting "musician" as an answer to "your current occupation" which appears on so many forms, from job applications/passports/loans/insurances etc.

Good thread - lots of points to mull over.