#apophony

• Ablaut — A systematic vowel alternation within the same root of a word, used in Indo-European languages to express differences in grammatical function, tense, mood, or lexical derivation, without adding suffixes. This mechanism is most visible in the inflection of irregular (strong) verbs in Germanic languages, where changing the vowel modifies the grammatical tense.
• Apophony — A regular and systematic modification of the vowel within a word root, fulfilling a grammatical function or serving lexical derivation, without being caused by neighboring sounds; independent vowel gradation or ablaut. The phenomenon is foundational in Indo-European and Semitic languages, a classic example being verb inflection in Germanic languages (such as the English sing/sang/sung alternation).