The Collaborative International Dictionary of English v.0.48:

Organist \Or"gan*ist\, n. [Cf. F. organiste.]
   1. (Mus.) One who plays on the organ.
      [1913 Webster]

   2. (R. C. Ch.) One of the priests who organized or sung in
      parts. [Obs.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

organist
    n 1: a person who plays an organ