n. 元音交替

[ noun ]
a vowel whose quality or length is changed to indicate linguistic distinctions (such as sing sang sung song)
<noun.communication>

Ablaut \Ab"laut\, n. [Ger., off-sound; ab off + laut sound.]
(Philol.)
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.