From The Art and Popular Culture Encyclopedia
In linguistics, an alternation is the phenomenon of a phoneme or morpheme exhibiting variation in its phonological realization. Each of the various realizations is called an alternant. The variation may be conditioned by the phonological, morphological, and/or syntactic environment in which the morpheme finds itself.
Alternations provide linguists with data that allow them to determine the allophones and allomorphs of a language's phonemes and morphemes and to develop analyses determining the distribution of those allophones and allomorphs.