In linguistics, apophony (also known as ablaut, gradation, 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).
Read more about Apophony: Description, Types, Stem Alternations and Other Morphological Processes, Transfixation, Replacive Morphemes, English Ablaut-motivated Compounding