onion vs veggie

onion and veggie both are nouns.

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

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

onion: an aromatic flavorful vegetable
veggie: edible seeds or roots or stems or leaves or bulbs or tubers or nonsweet fruits of any of numerous herbaceous plant

Other hypernyms of onion include veg, vegetable.

onion (noun)	veggie (noun)
an aromatic flavorful vegetable	edible seeds or roots or stems or leaves or bulbs or tubers or nonsweet fruits of any of numerous herbaceous plant
bulbous plant having hollow leaves cultivated worldwide for its rounded edible bulb
the bulb of an onion plant