green onion vs scallion

green onion and scallion both are nouns.

Word	Noun	Adjective	Verb	Adverb
green onion	Yes	No	No	No
scallion	Yes	No	No	No

As nouns, green onion and scallion are synonyms defined as:

green onion and scallion: a young onion before the bulb has enlarged; eaten in salads

Other synonyms of green onion include spring onion.

green onion (noun)	scallion (noun)
a young onion before the bulb has enlarged; eaten in salads	a young onion before the bulb has enlarged; eaten in salads
	plant having a large slender white bulb and flat overlapping dark green leaves; used in cooking; believed derived from the wild Allium ampeloprasum