Japanese leek vs Allium cepa

Japanese leek and Allium cepa both are nouns.

Word	Noun	Adjective	Verb	Adverb
Japanese leek	Yes	No	No	No
Allium cepa	Yes	No	No	No

As nouns, Allium cepa is a hypernym of Japanese leek; that is, Allium cepa is a word with a broader meaning than Japanese leek:

Japanese leek: Asiatic onion with slender bulbs; used as early green onions
Allium cepa: bulbous plant having hollow leaves cultivated worldwide for its rounded edible bulb

Other hypernyms of Japanese leek include onion, onion plant.

Japanese leek (noun)	Allium cepa (noun)
Asiatic onion with slender bulbs; used as early green onions	bulbous plant having hollow leaves cultivated worldwide for its rounded edible bulb