Meaning of cardinal vowel

car'dinal vow'el

Pronunciation: [key]
— Phonet. Phonet.
  1. any one of eight primary, purportedly invariant, sustained vowel sounds that constitute a reference set for describing the vowel inventory of a language.
  2. any one of the eight or more supplementary or secondary cardinal vowels.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.