spring onion vs onion

spring onion and onion both are nouns.

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

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

spring onion (noun)	onion (noun)
a young onion before the bulb has enlarged; eaten in salads	an aromatic flavorful vegetable
	bulbous plant having hollow leaves cultivated worldwide for its rounded edible bulb
	the bulb of an onion plant