Definition -
(1) A spoken sound in which the vocal tract is opened so that air pressure doesn't build up above the glottis, e.g. the [ei] in the word bait (pronounced /beit/). This is in contrast to consonants where the vocal tract is constricted.
(2) The written symbols used to indicate English vowel sounds (e.g. a, e, i, o, u, and sometimes y).

IPA vowel chart
IPA vowel chart

Etymology -
The word derives via Old French from the Latin vocalis (from the phrase in littera vocalis, vocal letter).

Oxford English Dictionary -
The word's first citation is from about 1308:
"Þis uers is imakid wel of consonans and wowel."
(Sat. Kildare iii. in E.E.P. (1862) 153 )

Quotation -
"A vowel may be defined as voice (voiced breath) modified by some definite configuration of the super-glottal passages, but without audible friction (which would make it into a consonant)."
(Source: Henrey Sweet, Primer of Phonetics, ed. 2, §32)

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