common fennel vs herb

common fennel and herb both are nouns.

Word	Noun	Adjective	Verb	Adverb
common fennel	Yes	No	No	No
herb	Yes	No	No	No

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

common fennel (noun)	herb (noun)
leaves used for seasoning	aromatic potherb used in cookery for its savory qualities
strongly aromatic with a smell of aniseed; leaves and seeds used for seasoning	a plant lacking a permanent woody stem; many are flowering garden plants or potherbs; some having medicinal properties; some are pests