I’ve noticed that the grocery stores where I shop no longer sell something called scallions. What they sell now are called green onions. If you know your onions, you know that green onions and scallions are not necessarily the same thing.
Green onions and spring onions should be immature Allium cepa (this is the botanical name for culinary onions).
Scallions, on the other hand, are either those certain forms of Allium cepa which do not bulb or (and this is where it gets confusing) forms of a related species Allium fistulosum, the so-called (i.e. misnamed) Welsh onion. This “Welsh onion” is actually native to Siberia.
Where do shallots fit into this picture? Many texts currently attribute shallots to Allium cepa as a “Group” called Aggregatum. A Group in this sense - notice the capital G - is an assemblage of forms which might or might not share the same origin but which share so many similarities that for practical purposes they are treated as essentially the same thing until someone parses their ancestry and interrelationships. This usage of Group is not taxonomic: these Groups do not fall into the traditional taxonomic hierarchy.
The old name for shallots was Allium ascalonicum. Ascalon was an ancient Mediterranean port city in what is now modern Israel. Notice the similarity between the words scallion and ascalonicum: it’s no accident. Scallion is ultimately derived from the old name ascalonicum. In English, it’s not a direct borrowing: the word had to travel through several languages before it made its way into English. You would therefore think that shallots are the rightful claimants to the name scallion, but I can’t recall any recent American text which calls shallots scallions.
After writing the above, I Googled shallot and read the wikipedia account. The writer of that account assigns the French gray shallot to Allium oschaninii and the other shallots to Allium cepa.
The plot thickens…