tuba vs brass instrument

tuba and brass instrument both are nouns.

Word	Noun	Adjective	Verb	Adverb
tuba	Yes	No	No	No
brass instrument	Yes	No	No	No

As nouns, brass instrument is a hypernym of tuba; that is, brass instrument is a word with a broader meaning than tuba:

tuba: the lowest brass wind instrument
brass instrument: a wind instrument that consists of a brass tube (usually of variable length) that is blown by means of a cup-shaped or funnel-shaped mouthpiece

Other hypernyms of tuba include bass, brass.

tuba (noun)	brass instrument (noun)
the lowest brass wind instrument	a wind instrument that consists of a brass tube (usually of variable length) that is blown by means of a cup-shaped or funnel-shaped mouthpiece