The Collaborative International Dictionary of English v.0.48:

Commentary \Com"men*ta*ry\, n.; pl. Commentaries. [L.
   commentarius, commentarium, note book, commentary: cf. F.
   commentaire. See Comment, v. i.]
   1. A series of comments or annotations; esp., a book of
      explanations or expositions on the whole or a part of the
      Scriptures or of some other work.
      [1913 Webster]

            This letter . . . was published by him with a severe
            commentary.                           --Hallam.
      [1913 Webster]

   2. A brief account of transactions or events written hastily,
      as if for a memorandum; -- usually in the plural; as,
      Caesar's Commentaries on the Gallic War.
      [1913 Webster]

WordNet (r) 3.0 (2006):

commentary
    n 1: a written explanation or criticism or illustration that is
         added to a book or other textual material; "he wrote an
         extended comment on the proposal" [syn: comment,
         commentary]