ablaut, n. [Ger., off-sound; ab off + laut sound.].
The substitution of one root vowel for another, thus indicating a corresponding modification of use or meaning; vowel permutation; as, get, gat, got; sing, song; hang, hung. Earle. [1913 Webster]
ablaut, n. a change of vowel in related words or forms, esp. in Indo-European languages, arising from differences of accent and stress in the parent language, e.g. in sing, sang, sung.