Vowel shift
A vowel shift is a systematic change in the pronunciation of the vowel sounds of a language.
The best-known example in the English language is the Great Vowel Shift, which began in the 15th century.
Other examples of ongoing vowel shifts are the Southern Vowel Shift in the southern United States, the California Vowel Shift in California English, or the Northern cities vowel shift in Michigan, Chicago, and parts of upstate New York.
The Greek language also underwent a vowel shift near the beginning of the Common Era.
A vowel shift can involve a merger of two previously different sounds, or it can be a chain shift, where vowels play "musical chairs".