A vowel change, characteristic of Indo-European languages, that accompanies a change in grammatical function; for example, i, a, u in sing, sang, sung. Also called apophony, gradation.
[German : ab, off (from Middle High German ab, abe, from Old High German aba; see apo- in Indo-European roots) + Laut, sound (from Middle High German lūt, from Old High German hlūt; see kleu- in Indo-European roots).]
(esp. in Indo-European languages) regular alternation of vowels in a word element, reflecting a change in grammatical function, as in English sing, sang, sung, song.
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