The process of repeatedly changing from one state or condition to another and back again. The alternation of day and night is a natural phenomenon.
A phonological or morphological variation in the form of a morpheme or phoneme that is conditioned by its linguistic environment. The vowel alternation in "sing", "sang", and "sung" illustrates a historical ablaut pattern.
One half-cycle of an alternating current measured between two successive zero crossings in the same direction. At 60 hertz, an AC circuit experiences 120 alternations each second.