Dictionary.net

• 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.

2010 - New Age Dictionary Database
By Oddity Software

• 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.

1913 - Webster's Revised Unabridged Dictionary
By Noah Webster.