scallion vs green onion

scallion and green onion both are nouns.

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

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

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

Other synonyms of scallion include spring onion.

scallion (noun)	green onion (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