The Collaborative International Dictionary of English v.0.48:

Gradation \Gra*da"tion\, n., [L. gradatio: cf. F. gradation. See
   Grade.]
   1. The act of progressing by regular steps or orderly
      arrangement; the state of being graded or arranged in
      ranks; as, the gradation of castes.
      [1913 Webster]

   2. The act or process of bringing to a certain grade.
      [1913 Webster]

   3. Any degree or relative position in an order or series.
      [1913 Webster]

            The several gradations of the intelligent universe.
                                                  --I. Taylor.
      [1913 Webster]

   4. (Fine Arts) A gradual passing from one tint to another or
      from a darker to a lighter shade, as in painting or
      drawing.
      [1913 Webster]

   6. (Mus.) A diatonic ascending or descending succession of
      chords.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Gradation \Gra*da"tion\, v. t.
   To form with gradations. [R.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

gradation
    n 1: relative position in a graded series; "always a step
         behind"; "subtle gradations in color"; "keep in step with
         the fashions" [syn: gradation, step]
    2: a degree of ablaut [syn: grade, gradation]
    3: the act of arranging in grades [syn: gradation,
       graduation]

Moby Thesaurus II by Grady Ward, 1.0:

97 Moby Thesaurus words for "gradation":
   Indian file, ablaut, acoustic phonetics, array, articulation,
   articulatory phonetics, assortment, bank, betacism, buzz,
   categorization, catena, catenation, chain, chain reaction,
   chaining, change, concatenation, connection, consecution,
   continuity, continuum, course, culling, cycle, degree, descent,
   difference, distinction, divergence, drone, endless belt,
   endless round, file, filiation, gamut, grading, graduation,
   grouping, hierarchy, hum, line, lineage, modification, monotone,
   morphophonemics, mutation, nexus, nuance, orthoepy, pendulum,
   periodicity, phonetics, phonology, place, placement, plenum,
   powder train, progression, queue, range, rank, ranking, recurrence,
   reticulation, rhotacism, rotation, round, routine, row, run, scale,
   screening, selection, sequence, serial order, series, shade,
   shading, sifting, single file, sorting, sorting out, sound shift,
   spectrum, string, subordination, succession, swath, taxonomy,
   thread, tier, train, triage, umlaut, variation, windrow