What is "apophony"

n. (context phonetics English) Alternation of sounds within a word that indicates grammatical information (often inflectional).

In linguistics, apophony (also known as ablaut, (vowel) gradation, (vowel) mutation, alternation, internal modification, stem modification, stem alternation, replacive morphology, stem mutation, internal inflection etc.) is the alternation of sounds within a word that indicates grammatical information (often inflectional).