The Collaborative International Dictionary of English v.0.48:

Alliteration \Al*lit`er*a"tion\, n. [L. ad + litera letter. See
   Letter.]
   The repetition of the same letter at the beginning of two or
   more words immediately succeeding each other, or at short
   intervals; as in the following lines:
   [1913 Webster]

         Behemoth, biggest born of earth, upheaved
         His vastness.                            --Milton.
   [1913 Webster]

         Fly o'er waste fens and windy fields.    --Tennyson.
   [1913 Webster]

   Note: The recurrence of the same letter in accented parts of
         words is also called alliteration. Anglo-Saxon poetry
         is characterized by alliterative meter of this sort.
         Later poets also employed it.
         [1913 Webster]

               In a somer seson whan soft was the sonne,
               I shope me in shroudes as I a shepe were. --P.
                                                  Plowman.
         [1913 Webster]

WordNet (r) 3.0 (2006):

alliteration
    n 1: use of the same consonant at the beginning of each stressed
         syllable in a line of verse; "around the rock the ragged
         rascal ran" [syn: alliteration, initial rhyme,
         beginning rhyme, head rhyme]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "alliteration":
   assonance, blank verse, chime, clink, consonance, crambo, dingdong,
   double rhyme, drone, eye rhyme, harping, humdrum, jingle,
   jingle-jangle, monotone, monotony, near rhyme, paronomasia,
   pitter-patter, pun, repeated sounds, repetitiousness,
   repetitiveness, rhyme, rhyme royal, rhyme scheme,
   rhyming dictionary, single rhyme, singsong, slant rhyme,
   stale repetition, tail rhyme, tedium, trot, unnecessary repetition,
   unrhymed poetry