In linguistics, a simulfix is a type of affix that changes one or more existing phonemes in order to modify the meaning of a morpheme.
Examples of simulfixes in English are generally considered irregularities, all of which left over from pluralization rules that existed before the Great Vowel Shift. They include:
The transfixes of the Semitic languages may be considered a form of discontinuous simulfix.
References
Simulfix Wikipedia(Text) CC BY-SA