The Collaborative International Dictionary of English v.0.48:

Polysyllable \Pol"y*syl`la*ble\, n. [Poly- + syllable.]
   A word of many syllables, or consisting of more syllables
   than three; -- words of less than four syllables being called
   monosyllables, dissyllables, and trisyllables.
   [1913 Webster]

WordNet (r) 3.0 (2006):

polysyllable
    n 1: a word of more than three syllables [syn: polysyllable,
         polysyllabic word]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "polysyllable":
   antonym, articulation, expression, free form, grandiloquence,
   hard word, homograph, homonym, homophone, jawbreaker, lexeme,
   lexiphanicism, linguistic form, locution, logos, long word,
   metonym, minimum free form, monosyllable, sesquipedalian, syllable,
   synonym, term, usage, utterance, verbalism, verbum, vocable,
   word