What is a Musician?

I’ve been looking at those new Gap ads around town – some skinny people (most of whom I’ve never heard of) labelled “actor or musician”. They look cool, but I don’t see any instruments and I’m wondering exactly who they are and what they do. Usually when our culture discusses a musician they’re referring to a rock star. Obviously there are many trained musicians who take up pop music. But is there a standard for calling someone a “musician”? Is Rod Stewart, who once acknowledged in an interview that he literally knows three chords on the guitar, a musician? What about a rapper who does not and cannot engage melody or rhythm? Is anyone who makes music a musician, or does being musician require a certain skill set, whether technical, intellectual or aural?

In your opinion, what is a musician? Please comment!