Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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