The Collaborative International Dictionary of English v.0.48:

Undulation \Un`du*la"tion\, n. [Cf. F. ondulation.]
   1. The act of undulating; a waving motion or vibration; as,
      the undulations of a fluid, of water, or of air; the
      undulations of sound.
      [1913 Webster]

   2. A wavy appearance or outline; waviness. --Evelyn.
      [1913 Webster]

   3. (Mus.)
      (a) The tremulous tone produced by a peculiar pressure of
          the finger on a string, as of a violin.
      (b) The pulsation caused by the vibrating together of two
          tones not quite in unison; -- called also beat.
          [1913 Webster]

   4. (Physics) A motion to and fro, up and down, or from side
      to side, in any fluid or elastic medium, propagated
      continuously among its particles, but with no translation
      of the particles themselves in the direction of the
      propagation of the wave; a wave motion; a vibration.
      [1913 Webster]

WordNet (r) 3.0 (2006):

undulation
    n 1: an undulating curve [syn: wave, undulation]
    2: wavelike motion; a gentle rising and falling in the manner of
       waves
    3: (physics) a movement up and down or back and forth [syn:
       wave, undulation]