open vowel vs vowel

open vowel and vowel both are nouns.

Word	Noun	Adjective	Verb	Adverb
open vowel	Yes	No	No	No
vowel	Yes	No	No	No

As nouns, vowel is a hypernym of open vowel; that is, vowel is a word with a broader meaning than open vowel:

open vowel: a vowel that you pronounce with your tongue on the bottom of your mouth.
vowel: a speech sound made with the vocal tract open

Other hypernyms of open vowel include vowel sound.

open vowel (noun)	vowel (noun)
a vowel that you pronounce with your tongue on the bottom of your mouth.	a speech sound made with the vocal tract open
	a letter of the alphabet standing for a spoken vowel