Webster's English Dictionary

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undulation

un.du.la.tion \.*n-j*-'la--sh*n, .*n-d(y)*-\ n 1a: a rising and falling in 
   waves  1b: a wavelike motion to and fro in a fluid or elastic medium 
   propagated co ntinuously among its particles but with little or no 
   permanent translation of the particles in the direction of the propagation 
   : VIBRATION 2: the pulsation caused by the vibrating together of two tones 
   not quite in  unison 3: a wavy appearance, outline, or form : WAVINESS