Definition - A vowel-sound change within a word that shows changed grammatical function.

Example -

(1) sing, sang, and sung
(2) mouse and mice

Etymology -
The word derives from the German Ablaut, off-sound; the latter was coined in 1568 by J.P. Zweigel (from ab, off + Laut, sound or tone).

Oxford English Dictionary -
Its first citation is from 1849:
"As if it took its meaning from ‘roar’; which in reality is the ablaut form of ‘rear’."
(E. Thomson Select Monuments p. xxxi, )

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