Source : Webster's Revised Unabridged Dictionary (1913)

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}.

Source : WordNet®

polysyllable
     n : a word of more than three syllables [syn: {polysyllabic word}]