ab·laut 
(äblout′, äp-)
(äblout′, äp-)n.
A vowel change, characteristic of Indo-European languages, that accompanies a change in grammatical function; for example, i, a, u in sing, sang, sung. Also called apophony, gradation.
[German : ab, off (from Middle High German ab, abe, from Old High German aba; see apo- in the Appendix of Indo-European roots) + Laut, sound (from Middle High German lūt, from Old High German hlūt; see kleu- in the Appendix of Indo-European roots).]