purple clover vs trefoil

purple clover and trefoil both are nouns.

Word	Noun	Adjective	Verb	Adverb
purple clover	Yes	No	No	No
trefoil	Yes	No	No	No

As nouns, trefoil is a hypernym of purple clover; that is, trefoil is a word with a broader meaning than purple clover:

purple clover: erect to decumbent short-lived perennial having red-purple to pink flowers; the most commonly grown forage clover
trefoil: a plant of the genus Trifolium

Other hypernyms of purple clover include clover.

purple clover (noun)	trefoil (noun)
erect to decumbent short-lived perennial having red-purple to pink flowers; the most commonly grown forage clover	an architectural ornament in the form of three arcs arranged in a circle
	a plant of the genus Trifolium
	any of several Old World herbs of the genus Medicago having small flowers and trifoliate compound leaves