Welsh onion vs onion

Welsh onion and onion both are nouns.

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

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

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

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

Welsh onion (noun)	onion (noun)
Asiatic onion with slender bulbs; used as early green onions	an aromatic flavorful vegetable
	bulbous plant having hollow leaves cultivated worldwide for its rounded edible bulb
	the bulb of an onion plant