/ahb"lowt, ab"-/ ; Ger. /ahp"lowt/ , n. Gram.
(in Indo-European languages) regular alternation in the internal phonological structure of a word element, esp. alternation of a vowel, that is coordinated with a change in grammatical function or combination, as in English sing, sang, sung, song; apophony.
[ 1840-50; ab- off + Laut sound ]