tree onion vs onion

tree onion and onion both are nouns.

Word	Noun	Adjective	Verb	Adverb
tree onion	Yes	No	No	No
onion	Yes	No	No	No

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

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

Other hypernyms of tree onion include Allium cepa, onion plant.

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