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