Allomorph

When a particular morpheme is not represented everywhere by the same morph, but by different morphs in different environments, these alternative representations of the morpheme are called allomorphs.

Examples
The plural morpheme in English is regularly represented by the allomorphs [s], [z] and [ɨz]. The rule is as follows:

(i) if the noun stem ends in a voiceless consonant, the allomorph used is [s], as in kicks [kɪks];

(ii) if the noun stem ends in a voiced phoneme (including a vowel), the allomorphed used is [z] (as in cats [kæts]);

(iii) if the noun stem ends with a sibilant ([s], [z] etc.), the allomorph used is [ɨz] (as in sizes [saizɨz]).

Other languages
German Allomorph (de) Chinese 语素变体