ablaut

from 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)

from 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]