long vowel
Definition - A vowel that is sounded with extended duration.

Example -
Traditionally, in English the long vowels are:
(1) [ei] (as in bait)
(2) [i:] (beet)
(3) [ai] (bite)
(4) [oʊ] (boat)
(5) [ju:] (beauty)
Note: This is based upon English pronunciation prior to the Great Vowel Shift, not on current pronunciation. Nowadays, most of the long vowels are classed as diphthongs

Oxford English Dictionary -
The term's first citation is from 1867:
"Salesbury does not always discriminate the long vowel, though … he occasionally … doubles the consonant sign to imply the brevity of the preceding vowel."
( Ellis E.E. Pronunc. i. iii. 61)

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