Scallions are vegetables derived from various species in the genus Allium.
Scallions have a milder taste than most onions.
Their close relatives include garlic, shallot, leek, chive, and Chinese onions.
Scallions and green onions are literally the same thing.
Spring onions, on the other hand, are a different thing.
The bulb of a spring onion is much larger, compared to the small, not-so-bulbous scallion.
Most scallions (and green onions) never grow true bulbs.
The ends of the stalks stay small.