What is "ablaut"

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.

"systematic vowel alteration in the root of a word to indicate shades of meaning or tense," a characteristic of Indo-European languages, 1849, from German Ablaut, literally "off-sound," coined by J.P. Zweigel in 1568 from ab "off" + Laut "sound, tone," from Old High German hlut (see listen). Popularized by Jacob Grimm.

n. (context linguistics English) The substitution of one root vowel for another, thus indicating a corresponding modification of use or meaning; vowel permutation; as, (term get English), (term gat English), and (term got English); (term sing English) and (term song English); (term hang English) and (term hung English), distinct from the phonetic influence of a succeeding vowel. (Mid 19^th century.)(R:SOED5: page=5) vb. 1 (context intransitive linguistics of a vowel-containing linguistic component English) To undergo a change of vowel. 2 (context transitive linguistics English) To cause to change a vowel.

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