Allomorphy

Allomorphy is the phenomenon that a single morpheme has different realizations, i.e. alternative forms depending on the phonological or morphological context in which it appears. In another type of allomorphy, the realization of a morpheme is conditioned by the presence of another morpheme.

Examples
In English, the plural suffix has three pronunciations:

(a) /s/ after nouns ending in a voiceless consonant (cats /kats/),

(b) /z/ after nouns ending in a voiced consonant (dogs /dogz/), and

(c) /ɪz/ after nouns ending in a coronal sibilant (horses /horsɪz/). The English suffix -able is pronounced /ɪbl/ in adjectives such as possible and probable, but when the noun-forming suffix -ity is attached to it it is pronounced as /ɪbil/ (possibility, probability).

Link
Utrecht Lexicon of Linguistics