Definition - The varying of vowels to form grammatically related words.

Example -
sing, sang, and sung

Etymology -
The word derives from the Greek apo, away from + phone, sound.

Oxford English Dictionary -
Its first citation is from 1883:
"The Germans call this … Ablaut, that may be rendered by apophony."
(I. Sydow, German Conversational Grammar, p. 253)

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