The Collaborative International Dictionary of English v.0.48:

Monotone \Mon"o*tone\, n. [See Monotonous, Monotony.]
   1. (Mus.) A single unvaried tone or sound.
      [1913 Webster]

   2. (Rhet.) The utterance of successive syllables, words, or
      sentences, on one unvaried key or line of pitch.
      [1913 Webster] Monotonic

WordNet (r) 3.0 (2006):

monotone
    adj 1: of a sequence or function; consistently increasing and
           never decreasing or consistently decreasing and never
           increasing in value [syn: monotonic, monotone] [ant:
           nonmonotonic]
    2: sounded or spoken in a tone unvarying in pitch; "the owl's
       faint monotonous hooting" [syn: flat, monotone,
       monotonic, monotonous]
    n 1: an unchanging intonation [syn: monotone, drone,
         droning]
    2: a single tone repeated with different words or different
       rhythms (especially in rendering liturgical texts)

Moby Thesaurus II by Grady Ward, 1.0:

134 Moby Thesaurus words for "monotone":
   AF, Indian file, alliterating, alliteration, alliterative, array,
   articulation, assonance, assonant, audio frequency, banausic, bank,
   belabored, blah, buzz, catena, catenation, chain, chain reaction,
   chaining, chanting, chime, chiming, cliche-ridden,
   clockwork regularity, concatenation, connection, consecution,
   constancy, continuum, course, cycle, daily round, descent, dim,
   dingdong, dreary, drone, droning, endless belt, endless round,
   even pace, even tenor, file, filiation, frequency, fundamental,
   fundamental tone, gamut, gradation, harmonic, harping, hum,
   humdrum, intonation, invariability, jingle, jingle-jangle,
   jog-trot, labored, line, lineage, monologue, monotonic, monotonous,
   monotonousness, monotony, near rhyme, nexus, orderliness, overtone,
   partial, partial tone, pedestrian, pendulum, periodicity, pitch,
   pitter-patter, plenum, poky, powder train, progression, queue,
   range, rank, recurrence, regularity, repeated sounds,
   repetitiousness, repetitiveness, reticulation, rhyme, rhymed,
   rhyming, rotation, round, routine, row, run, sameliness, sameness,
   scale, sequence, series, single file, singsong, slant rhyme,
   smoothness, soniferous, sonorous, sounded, sounding, spectrum,
   stale repetition, stodgy, string, succession, swath, tedious,
   tedium, thread, tier, tonal, tone, toneless, tonelessness, train,
   treadmill, trot, undeviation, undifferentiation,
   unnecessary repetition, unvariation, windrow