long vowel vs vowel

long vowel and vowel both are nouns.

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

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

long vowel: A vowel which is normally pronounced somewhat longer than other vowels (usually around 1½ to double length).
vowel: a speech sound made with the vocal tract open

Other hypernyms of long vowel include vowel sound.

long vowel (noun)	vowel (noun)
A vowel which is normally pronounced somewhat longer than other vowels (usually around 1½ to double length).	a speech sound made with the vocal tract open
	a letter of the alphabet standing for a spoken vowel