Apophony

Apophony is a technical term in linguistics. (Say 'er-POFF-er-ny, IPA: /ə ˈpɒf ən ɪ/.) It means 'a change in vowel sound to indicate a different grammatical function', as in plurals of some irregular nouns like 'goose' → 'geese' and 'child' (IPA: /tʃaɪld/) → 'children' ((IPA: /ˈtʃɪld rən/), and in irregular verbs like 'drink' (present), 'drank' (past tense), 'drunk' (past participle.

Apophony is often divided into

• ablaut, which is apophony by variation of a root vowel, most noticeably in Germanic strong verbs. This can be labelled by the English term 'vowel gradation'
• umlaut, which is a change is a vowel sound, partly by assimilation to another vowel nearby. This can be called 'mutation'.