|Sound change and alternation|
In linguistics, an alternation is the phenomenon of a 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.
The term "sound change" refers to diachronic changes, which occur in a language's sound system. On the other hand, "alternation" refers to changes that happen synchronically (within the language of an individual speaker, depending on the neighbouring sounds) and do not change the language's underlying system.