The Collaborative International Dictionary of English v.0.48:

Commentator \Com"men*ta`tor\, n. [L. commentator: cf. F.
   commentateur.]
   One who writes a commentary or comments; an expositor; an
   annotator.
   [1913 Webster]

         The commentator's professed object is to explain, to
         enforce, to illustrate doctrines claimed as true.
                                                  --Whewell.
   [1913 Webster]

WordNet (r) 3.0 (2006):

commentator
    n 1: an expert who observes and comments on something [syn:
         observer, commentator]
    2: a writer who reports and analyzes events of the day [syn:
       commentator, reviewer]