The Collaborative International Dictionary of English v.0.48:

Assonance \As"so*nance\, n. [Cf. F. assonance. See Assonant.]
   1. Resemblance of sound. "The disagreeable assonance of
      `sheath' and `sheathed.'" --Steevens.
      [1913 Webster]

   2. (Pros.) A peculiar species of rhyme, in which the last
      accented vowel and those which follow it in one word
      correspond in sound with the vowels of another word, while
      the consonants of the two words are unlike in sound; as,
      calamo and platano, baby and chary.
      [1913 Webster]

            The assonance is peculiar to the Spaniard. --Hallam.
      [1913 Webster]

   3. Incomplete correspondence.
      [1913 Webster]

            Assonance between facts seemingly remote. --Lowell.
      [1913 Webster]

WordNet (r) 3.0 (2006):

assonance
    n 1: the repetition of similar vowels in the stressed syllables
         of successive words [syn: assonance, vowel rhyme]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "assonance":
   alliteration, 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