a series of changes in the quality of the long vowels between Middle and Modern English as a result of which all were raised, while the high vowels (ē) and (o̅o̅), already at the upper limit, underwent breaking to become the diphthongs (ī) and (ou).
British Dictionary definitions for Great Vowel Shift
Great Vowel Shift
(linguistics) a phonetic change that took place during the transition from Middle to Modern English, whereby the long vowels were raised: (eː became iː , oː became uː), etc. The vowels (iː) and (uː) underwent breaking and became the diphthongs (aɪ) and (aʊ)