water horehound vs herb

water horehound and herb both are nouns.

Word	Noun	Adjective	Verb	Adverb
water horehound	Yes	No	No	No
herb	Yes	No	No	No

As nouns, herb is a hypernym of water horehound; that is, herb is a word with a broader meaning than water horehound:

water horehound: aromatic perennial herb of United States
herb: a plant lacking a permanent woody stem; many are flowering garden plants or potherbs; some having medicinal properties; some are pests

Other hypernyms of water horehound include herbaceous plant.

water horehound (noun)	herb (noun)
aromatic perennial herb of United States	aromatic potherb used in cookery for its savory qualities
	a plant lacking a permanent woody stem; many are flowering garden plants or potherbs; some having medicinal properties; some are pests