Meaning of Great Vowel Shift

Great' Vow'el Shift'

Pronunciation: [key]
— Ling. Ling.
  1. 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 (&oomacr;), already at the upper limit, underwent breaking to become the diphthongs (ī) and (ou).
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.