Source : Webster's Revised Unabridged Dictionary (1913)

Monosyllable \Mon"o*syl`la*ble\, n. [L. monosyllabus of one
   syllable, Gr. ?: cf. F. monosyllabe. See {Mono-},
   {Syllable}.]
   A word of one syllable.

Source : WordNet®

monosyllable
     n : a word or utterance of one syllable [syn: {monosyllabic word}]