viol vs bass viol

viol and bass viol both are nouns.

Word	Noun	Adjective	Verb	Adverb
viol	Yes	No	No	No
bass viol	Yes	No	No	No

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

viol: any of a family of bowed stringed instruments that preceded the violin family
bass viol: viol that is the bass member of the viol family with approximately the range of the cello

Other hyponyms of viol include viola da braccio, gamba, viola da gamba, viola d'amore.

viol (noun)	bass viol (noun)
any of a family of bowed stringed instruments that preceded the violin family	viol that is the bass member of the viol family with approximately the range of the cello
	largest and lowest member of the violin family