"(1) between two instantaneous values of the same sinusoidal quantity: the fraction of the whole period that elapses between their occurrence.
(2) betwen two sinusoidal quantities having the same frequency: the fraction of the whole period taht elapses between the occurence of an instantaneous value of one and the instantaneous value of the other at the corresponding point of the cycle. By antural extension, the concepts of both phase and phase difference are applied to non-sinusoidal periodic quantities, even such as the left and right leg movements in locomotion or stridulation. These are symphasic, or in phase, when there is no phase difference, and the legs are moving in perfect unison; and antiphasic, or fully out of phase, when the phase difference is half a period and the legs are moving in exact anithesis." 
- . Glossarial Index. In: Acoustic Behavior of Animals. Acoustic Behavior of Animals. Elsevier; 1963.
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