The Collaborative International Dictionary of English v.0.48:

Monosyllable \Mon"o*syl`la*ble\, n. [L. monosyllabus of one
   syllable, Gr. ?: cf. F. monosyllabe. See Mono-,
   Syllable.]
   A word of one syllable.
   [1913 Webster]

WordNet (r) 3.0 (2006):

monosyllable
    n 1: a word or utterance of one syllable [syn: monosyllable,
         monosyllabic word]

Moby Thesaurus II by Grady Ward, 1.0:

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