tuba vs euphonium

tuba and euphonium both are nouns.

Word	Noun	Adjective	Verb	Adverb
tuba	Yes	No	No	No
euphonium	Yes	No	No	No

As nouns, euphonium is a hyponym of tuba; that is, euphonium is a word with a more specific, narrower meaning than tuba:

tuba: the lowest brass wind instrument
euphonium: a bass horn (brass wind instrument) that is the tenor of the tuba family

Other hyponyms of tuba include bombardon, helicon.

tuba (noun)	euphonium (noun)
the lowest brass wind instrument	a bass horn (brass wind instrument) that is the tenor of the tuba family