Wiktionary
vowel harmony
n. (context linguistics English) A phonological process involving vowels in some languages, setting constraints on what vowels (e.g. front vowel/back vowel only) may be found near each other and sometimes in the entire word.
Wikipedia
Vowel harmony
Vowel harmony is a type of long-distance assimilatory phonological process involving vowels that occurs in some languages. In languages with vowel harmony, there are constraints on which vowels may be found near each other. Many agglutinative languages have vowel harmony.
Usage examples of "vowel harmony".
The problems attending their explanation in English and in Welsh are similar (for instance, the question of the varying parts played by anticipation or 'vowel harmony' and by epenthesis), and the study of them together throws light on both.