Webster's English Dictionary

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ablaut

ab.laut \'a:p-.lau.t, 'ab-\ n [G, fr. ab away from + laut sound] : a 
   systematic variation of vowels in the same root or affix or in related  
   roots or affixes esp. in the Indo-european languages that is usu. 
   paralleled by differences in use or meaning (as in sing, sang, sung, song)