The Collaborative International Dictionary of English v.0.48:

Diction \Dic"tion\, n. [L. dicto a saying, a word, fr. dicere,
   dictum, to say; akin to dicare to proclaim, and to E. teach,
   token: cf. F. diction. See Teach, and cf. Benison,
   Dedicate, Index, Judge, Preach, Vengeance.]
   Choice of words for the expression of ideas; the
   construction, disposition, and application of words in
   discourse, with regard to clearness, accuracy, variety, etc.;
   mode of expression; language; as, the diction of Chaucer's
   poems.
   [1913 Webster]

         His diction blazes up into a sudden explosion of
         prophetic grandeur.                      --De Quincey.

   Syn: Diction, Style, Phraseology.

   Usage: Style relates both to language and thought; diction,
          to language only; phraseology, to the mechanical
          structure of sentences, or the mode in which they are
          phrased. The style of Burke was enriched with all the
          higher graces of composition; his diction was varied
          and copious; his phraseology, at times, was careless
          and cumbersome. "Diction is a general term applicable
          alike to a single sentence or a connected composition.
          Errors in grammar, false construction, a confused
          disposition of words, or an improper application of
          them, constitute bad diction; but the niceties, the
          elegancies, the peculiarities, and the beauties of
          composition, which mark the genius and talent of the
          writer, are what is comprehended under the name of
          style." --Crabb.
          [1913 Webster]

WordNet (r) 3.0 (2006):

diction
    n 1: the articulation of speech regarded from the point of view
         of its intelligibility to the audience [syn: enunciation,
         diction]
    2: the manner in which something is expressed in words; "use
       concise military verbiage"- G.S.Patton [syn: wording,
       diction, phrasing, phraseology, choice of words,
       verbiage]

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "diction":
   articulation, delivery, elocution, enunciation, expression,
   expressiveness, inflection, intonation, language, oratory,
   parlance, phrase, phraseology, phrasing, presentation,
   pronunciation, rhetoric, speech, terminology, usage, verbalism,
   verbiage, vocabulary, wordage, wording