Wordnet 3.0
NOUN (1)
1.
a vowel whose quality or length is changed to indicate linguistic distinctions (such as sing sang sung song);
The Collaborative International Dictionary of English v.0.48:
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.
[1913 Webster]
WordNet (r) 3.0 (2006):
ablaut
n 1: a vowel whose quality or length is changed to indicate
linguistic distinctions (such as sing sang sung song)