#undulation__wave  (physics) a progressive disturbance propagated without displacement of the medium itself
  supertype:  periodic_event__recurrent_event__recurrentevent  an event that recurs at intervals
  subtype:  gravity_wave  sine_wave  vibration  fluctuation  seiche  standing_wave  sound_wave  waveform  pulsing  flapping
  part:  wave_front  all the points just reached by a wave as it propagates

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