cardinal vowels

noun Phonetics.

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.

pl n

1. a set of theoretical vowel sounds, based on the shape of the mouth needed to articulate them, that can be used to classify the vowel sounds of any speaker in any language