The Collaborative International Dictionary of English v.0.48:

Rhythm \Rhythm\, n. [F. rhythme, rythme, L. rhythmus, fr. Gr.
   ??? measured motion, measure, proportion, fr. "rei^n to flow.
   See Stream.]
   1. In the widest sense, a dividing into short portions by a
      regular succession of motions, impulses, sounds, accents,
      etc., producing an agreeable effect, as in music poetry,
      the dance, or the like.
      [1913 Webster]

   2. (Mus.) Movement in musical time, with periodical
      recurrence of accent; the measured beat or pulse which
      marks the character and expression of the music; symmetry
      of movement and accent. --Moore (Encyc.)
      [1913 Webster]

   3. A division of lines into short portions by a regular
      succession of arses and theses, or percussions and
      remissions of voice on words or syllables.
      [1913 Webster]

   4. The harmonious flow of vocal sounds.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rhythm
    n 1: the basic rhythmic unit in a piece of music; "the piece has
         a fast rhythm"; "the conductor set the beat" [syn:
         rhythm, beat, musical rhythm]
    2: recurring at regular intervals [syn: rhythm, regular
       recurrence]
    3: an interval during which a recurring sequence of events
       occurs; "the never-ending cycle of the seasons" [syn:
       cycle, rhythm, round]
    4: the arrangement of spoken words alternating stressed and
       unstressed elements; "the rhythm of Frost's poetry" [syn:
       rhythm, speech rhythm]
    5: natural family planning in which ovulation is assumed to
       occur 14 days before the onset of a period (the fertile
       period would be assumed to extend from day 10 through day 18
       of her cycle) [syn: rhythm method of birth control, rhythm
       method, rhythm, calendar method of birth control,
       calendar method]

Moby Thesaurus II by Grady Ward, 1.0:

141 Moby Thesaurus words for "rhythm":
   Alexandrine, accent, accentuation, alternation, amphibrach,
   amphimacer, anacrusis, anapest, antispast, arrhythmia, arsis,
   bacchius, balance, beat, beating, beauty, cadence, cadency,
   caesura, catalexis, chloriamb, chloriambus, colon, concinnity,
   counterpoint, cretic, cyclicalness, dactyl, dactylic hexameter,
   diaeresis, dimeter, dipody, dochmiac, downbeat, drumming, elegiac,
   elegiac couplet, elegiac pentameter, emphasis, epitrite,
   equilibrium, euphony, feminine caesura, flutter, foot, harmony,
   heartbeat, heartthrob, heptameter, heptapody, heroic couplet,
   hexameter, hexapody, iamb, iambic, iambic pentameter, ictus,
   intermittence, intermittency, ionic, jingle, level of stress, lilt,
   masculine caesura, measure, measuredness, meter, metrical accent,
   metrical foot, metrical group, metrical unit, metrics, metron,
   molossus, mora, movement, number, numbers, order, orderedness,
   oscillation, paeon, palpitation, pendulum motion, pentameter,
   pentapody, period, periodicalness, periodicity, piston motion,
   pitapat, pitter-patter, primary stress, proceleusmatic, proportion,
   prosodics, prosody, pulsation, pulse, pyrrhic, quantity, rat-a-tat,
   rataplan, reappearance, recurrence, regular wave motion,
   reoccurrence, return, rhyme, rhythmic pattern, rhythmical stress,
   seasonality, secondary stress, spondee, sprung rhythm, staccato,
   stress, stress accent, stress pattern, sweetness, swing, symmetry,
   syzygy, tempo, tertiary stress, tetrameter, tetrapody, tetraseme,
   thesis, throb, throbbing, time, timing, tribrach, trimeter,
   tripody, triseme, trochee, undulation, upbeat, weak stress