apophony

Etymology
From, from French + , after Ancient Greek.

Noun

 * 1)  Modification of an internal vowel sound in a word or linguistic root, especially so as to establish a lexical distinction.

Usage notes
Apophony is often used synonymously with ablaut, but recent sources sometimes distinguish apophony as a lexical rather than grammatical feature: thus the words tip and top exhibit apophony, and tip-top is an apophonic reduplication.