Apophony

• A vowel whose quality or length is changed to indicate linguistic distinctions (such as sing sang sung song)
• Vowel change in related words

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
"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," ...

Wiktionary Word definitions in Wiktionary
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 ...

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

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
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.