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.

Phonologically conditioned alternation

An example of a phonologically conditioned alternation is the English plural marker commonly spelled s or es. This morpheme is pronounced, , or , depending on the nature of the preceding sound.

Alternation related to meaning

Morphologically conditioned alternation

French has an example of morphologically conditioned alternation. The feminine form of many adjectives ends in a consonant sound that is missing in the masculine form. In spelling, the feminine ends in a silent e, while the masculine ends in a silent consonant letter:

Syntactically conditioned alternation

Syntactically conditioned alternations can be found in the Insular Celtic languages, where words undergo various initial consonant mutations depending on their syntactic position. For example, in Irish, an adjective undergoes lenition after a feminine singular noun: In Welsh, a noun undergoes soft mutation when it is the direct object of a finite verb: