apophony

This term is a specialized linguistic descriptor used primarily in academic, philological, and phonetic contexts. It describes a systematic internal change in a word's vowel sound to signal a shift in meaning or grammatical category, such as the distinction between present and past tense in strong verbs. It is a more technical synonym for ablaut.

Because it refers to a specific linguistic phenomenon or process, it is treated as an uncountable noun. It describes the general mechanism of vowel alternation rather than individual instances of change, which are typically referred to as vowel shifts or alternations.