achillea vs milfoil

achillea and milfoil both are nouns.

Word	Noun	Adjective	Verb	Adverb
achillea	Yes	No	No	No
milfoil	Yes	No	No	No

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

achillea: any of several plants of the genus Achillea native to Europe and having small white flowers in flat-topped flower heads
milfoil: ubiquitous strong-scented mat-forming Eurasian herb of wasteland, hedgerow or pasture having narrow serrate leaves and small usually white florets; widely naturalized in North America

Other hyponyms of achillea include Achillea millefolium, yarrow, Achillea ptarmica, sneezeweed yarrow, sneezewort.

achillea (noun)	milfoil (noun)
any of several plants of the genus Achillea native to Europe and having small white flowers in flat-topped flower heads	ubiquitous strong-scented mat-forming Eurasian herb of wasteland, hedgerow or pasture having narrow serrate leaves and small usually white florets; widely naturalized in North America