It's a regional preference on what they're called.
When you're buying seeds, they're also called "bunching onions", and I grew up calling them "scallions", although I think that scallion specifically don't have a bulb yet formed, while green/spring onions might.
update : A little research suggests that "spring onion" is the preferred term in the UK, AU and Canada, "green onion" in the US south, and "scallion" in the US northeast, however I'm not sure what the exact boundries are as in the US mid-atlantic, I'll see both "green onions" and "scallions" for sale, with "green onions" being larger (scallions with no bulb, maybe 12"/30cm long, while green onions might have a 2"/5cm bulb, and have over 24"/60cm of green top). It's possible that "green onion" might be a polysemous term that varies by region.
Escallion = scallion = green onion.
I think it's possible to substitute regular onions in some situations, but there are a lot of variables.
First off, scallions have a sharper, more grassy flavor than onions, though they're not as strong.
Then there's the question of which kind of onion you want to substitute. White onions have some of that same sharp flavor, and are quite strong. Yellow onions are much milder and sweeter, and might not be such a close flavor match. Red onions are somewhere in between, in my experience. I would probably try white onion, but in much smaller quantity than called for of scallions.
It's also important to consider how the scallions are being used. If they're being cooked into the recipe, that's very different than if they're being added right at the end, or even as a garnish. I think the substitution is less of an issue if the scallions are to be cooked. If they're to be more or less raw, I'd suggest white onion, not too much of it, and sliced very very thinly, as a raw white onion is really strong.
Whatever you try, bear in mind that it won't be exactly the same. But that certainly doesn't mean it'll be bad--just different.
Best Answer
In general, shallots, leeks, or chives would be milder than regular onions and probably a closer approximation to green onions, though all are usually harder to find than green onions. Leeks and chives have the advantage that they also have a similar color to green onions. The sharpness of all of these substitutes (onions included) can be tamed by cooking, but this would affect both texture and color as well. In crab cakes, the color probably matters less than the flavor, but you could mix cooked shallots or onions with raw leeks or chives to get both color and flavor.