top onion vs Allium cepa

top onion and Allium cepa both are nouns.

Word	Noun	Adjective	Verb	Adverb
top onion	Yes	No	No	No
Allium cepa	Yes	No	No	No

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

top onion: type of perennial onion grown chiefly as a curiosity or for early salad onions; having bulbils that replace the flowers
Allium cepa: bulbous plant having hollow leaves cultivated worldwide for its rounded edible bulb

Other hypernyms of top onion include onion, onion plant.

top onion (noun)	Allium cepa (noun)
type of perennial onion grown chiefly as a curiosity or for early salad onions; having bulbils that replace the flowers	bulbous plant having hollow leaves cultivated worldwide for its rounded edible bulb