The Collaborative International Dictionary of English v.0.48:

Onomatopoeia \On`o*mat`o*poe"ia\, n. [L., fr. Gr. ?; 'o`noma,
   'ono`matos, a name + poiei^n to make.] (Philol.)
   The formation of words in imitation of sounds; a figure of
   speech in which the sound of a word is imitative of the sound
   of the thing which the word represents; as, the buzz of bees;
   the hiss of a goose; the crackle of fire.
   [1913 Webster]

   Note: It has been maintained by some philologist that all
         primary words, especially names, were formed by
         imitation of natural sounds, but this is not believed
         by most.
         [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

onomatopoeia
    n 1: using words that imitate the sound they denote

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "onomatopoeia":
   bowwow theory, copying, counterfeiting, echoic word, emulation,
   fakery, following, forgery, hit-off, imitation, impersonation,
   imposture, impression, mimesis, mirroring, onomatope,
   onomatopoeic word, parody, plagiarism, plagiary, repetition,
   simulation, takeoff