cuckoo flower vs lychnis

cuckoo flower and lychnis both are nouns.

Word	Noun	Adjective	Verb	Adverb
cuckoo flower	Yes	No	No	No
lychnis	Yes	No	No	No

As nouns, lychnis is a hypernym of cuckoo flower; that is, lychnis is a word with a broader meaning than cuckoo flower:

cuckoo flower: common perennial native to Europe and western Asia having usually pink flowers with ragged petals
lychnis: mostly perennial herbs with sticky stems that catch insects; widespread in north temperate zone

Other hypernyms of cuckoo flower include catchfly.

cuckoo flower (noun)	lychnis (noun)
common perennial native to Europe and western Asia having usually pink flowers with ragged petals	mostly perennial herbs with sticky stems that catch insects; widespread in north temperate zone
a bitter cress of Europe and America