belt bag vs fanny pack

belt bag and fanny pack both are nouns.

Word	Noun	Adjective	Verb	Adverb
belt bag	Yes	No	No	No
fanny pack	Yes	No	No	No

As nouns, fanny pack is a hyponym of belt bag; that is, fanny pack is a word with a more specific, narrower meaning than belt bag:

belt bag: a small pouch (usually with a zipper) that attaches to a belt and is worn around the waist
fanny pack: a waist pack worn with the pouch in back

Other hyponyms of belt bag include butt pack.

belt bag (noun)	fanny pack (noun)
a small pouch (usually with a zipper) that attaches to a belt and is worn around the waist	a waist pack worn with the pouch in back