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]