Chambers Twentieth Century Dictionary defines musidan as follows:
one skilled in music: a performer or composer of music, esp. professional
I have my own opinions on that last bit. As far as the old saying "you're born with it"...I believe that to be true. My earliest memories are entwined with the joy I felt when I heard or made melodic sounds. I actually whistled tunes before the age of two without being taught, simply because it made me feel good. I think what makes one feel "good" defines what one is.
I've been told I am not a musician because I don't know theory. I'm a bright individual and could probably learn theory if I wanted...but that takes the joy of music right out of me...like hitting the doldrums when sailing madly over the waves.