diphthong vs vowel

diphthong and vowel both are nouns.

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

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

diphthong: a vowel sound that starts near the articulatory position for one vowel and moves toward the position for another
vowel: a speech sound made with the vocal tract open

Other hypernyms of diphthong include vowel sound.

diphthong (noun)	vowel (noun)
a vowel sound that starts near the articulatory position for one vowel and moves toward the position for another	a speech sound made with the vocal tract open
	a letter of the alphabet standing for a spoken vowel