Hard question, but at the very least is someone who plays an instrument or sings (an audio technician or sampler can't be called musicians in my opinion, because they do not create music). I'd say there should be some level of proficiency involved in the term. It might include someone who writes songs, but I don't see that as definitive. A beginner musician should be called that, an amateur or hobbyist should be called that. A musician is, I think, a profession.
I do agree that a musician should aim to be an artist as well; meaning, he or she should create some of the music he/ she's playing. Even in covers, a musician could well have the ability to modify the original song to his/her own tastes. The thing is to give some feel to the playing, otherwise is purely technical and can't be considered artistic. In my opinion. Besides, music is an art, and an art is used to express oneself. So, a musician should seek to express him/ herself with the music.
Still, it is hard to come up with one answer for this. I guess the definition depends on the person.
Not a musician, but an amateur musician.