Nonconcatenative morphology

Nonconcatenative morphology, also called discontinuous morphology and introflection, is a form of word formation and inflection in which the root is modified and which does not involve stringing morphemes together sequentially. == Types == === Apophony (including Ablaut and Umlaut) === In English, for example, while plurals are usually formed by adding the suffix -s, certain words use nonconcatenative processes for their plural forms: foot /fʊt/ → feet /fiːt/; Many irregular verbs form their past tenses, past participles, or both in this manner: freeze /ˈfriːz/ → froze /ˈfroʊz/, frozen /ˈfroʊzən/.

Source: Wikipedia — Nonconcatenative morphology (CC BY-SA 4.0)