The Collaborative International Dictionary of English v.0.48:

Monotony \Mo*not"o*ny\, n. [Gr. ?: cf. F. monotonie. See
   Monotonius.]
   1. A frequent recurrence of the same tone or sound, producing
      a dull uniformity; absence of variety, as in speaking or
      singing.
      [1913 Webster]

   2. Any irksome sameness, or want of variety.
      [1913 Webster]

            At sea, everything that breaks the monotony of the
            surrounding expanse attracts attention. --W. Irving.
      [1913 Webster]

WordNet (r) 3.0 (2006):

monotony
    n 1: the quality of wearisome constancy, routine, and lack of
         variety; "he had never grown accustomed to the monotony of
         his work"; "he was sick of the humdrum of his fellow
         prisoners"; "he hated the sameness of the food the college
         served" [syn: monotony, humdrum, sameness]
    2: constancy of tone or pitch or inflection

Moby Thesaurus II by Grady Ward, 1.0:

91 Moby Thesaurus words for "monotony":
   AF, alliteration, assonance, audio frequency, boredom,
   broken record, ceaselessness, chime, clockwork regularity,
   consecutiveness, constancy, constant flow, continualness,
   continuity, continuousness, daily round, dingdong, drone, dryness,
   endlessness, ennui, equilibrium, even pace, even tenor,
   featurelessness, flatness, frequency, fullness, fundamental,
   fundamental tone, gaplessness, harmonic, harping, humdrum,
   incessancy, intonation, invariability, irk, irksomeness, jingle,
   jingle-jangle, jointlessness, monologue, monotone, monotonousness,
   near rhyme, orderliness, overtone, partial, partial tone, pitch,
   pitter-patter, plenitude, regularity, repeated sounds,
   repetitiousness, repetitiveness, rhyme, sameliness, sameness,
   seamlessness, singsong, slant rhyme, smoothness, stability,
   stale repetition, steadiness, steady state, successiveness, tedium,
   the beaten track, the daily round, the round, the squirrel cage,
   the treadmill, the weary round, tone, tonelessness, treadmill,
   trot, unbrokenness, undeviation, undifferentiation, uniformity,
   unintermittedness, uninterruptedness, uninterruption,
   unnecessary repetition, unrelievedness, unvariation,
   wearisome sameness