"The number of cycles or oscillations per unit time in a periodic system." 
"The repetition rate of the cycles of an oscillation (e.g. a sound wave). The frequency of audible sound waves is perceived by human ears as the pitch of the sound." 
"The rate of repetition of the cycles in a periodic quantity. The reciprocal of the period. The unit is the cycle per second (in Europe, the Hertz: 1 c/s = 1 Hz). By extension applied to recurring quantities even when successive cycles are not identical. In bio-acoustics, there is some tendency to reserve the term for the spectral components opf the elementary wave-form alone, a useful expedient for avoiding the all-too-easy confusion of these with other periodicities (such as the tooth-impact rate); periodicity and rate themselves are convenient terms for these latter." 
- . Arthropod Bioacoustics: Neurobiology and Behaviour. Ithaca, New York: Comstock; 1989.
- . The Songs of the Grasshoppers and Crickets of Western Europe. Colchester, Essex: Harley Books; 1998.
- . Glossarial Index. Acoustic Behavior of Animals. Acoustic Behavior of Animals. Elsevier; 1963.
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