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From 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]