[1] : 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

[2] : WordNet (r) 2.0

ablaut
     n : a vowel whose quality or length is changed to indicate
         linguistic distinctions such as sing sang sung song

Ablaut >> A Words
Ablaut, definition of term: Ablaut
ablaut_pag1.html