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]