In linguistics, an alternation is the phenomenon of a morpheme exhibiting variation in its phonological realization.
The variation may be conditioned by the phonological, morphological, and/or syntactic environment in which the morpheme finds itself.
An example of a phonologically conditioned alternation is the English plural marker commonly spelled s or es.
[1] This morpheme is pronounced /s/, /z/, or /ᵻz/,[note 1] depending on the nature of the preceding sound.
In spelling, the feminine ends in a silent e, while the masculine ends in a silent consonant letter:[2] Syntactically conditioned alternations can be found in the Insular Celtic languages, where words undergo various initial consonant mutations depending on their syntactic position.