Scallions (Allium fistulosum L., Syn.: A. altaicum, . ceratophyllum, Cepa sissilis, C. ventricosa) are often called green onions or welsh onions. There are also many regional names including spring onions, table onions, salad onions, onion sticks, long onions, baby onions, precious onions, yard onions, gibbons, syboes, and scally onions. Scallions are very similar to bulb onions. However, they have a less intense taste (apart from the darker green tops). Both the green and white parts of the stem can be eaten raw or cooked.
Scallions are very easy to use. You don’t have to peel them, and it is enough to simply rinse them off with water. Then you can either cut them into thin or thick rings. The green part of the stem works well as an ingredient for soups and salads. If they are freshly chopped, scallions can also be used as a substitute for dill. You will find scallions in a number of different types of dishes, but they are especially popular in Asian cuisine. The white part of the stem shouldn’t be sautéed or cooked at too high of temperatures as it quickly loses flavor when heated.
Since scallions wilt quickly, you should use them as soon as possible. They are more delicate and have a much shorter shelf life as compared to bulb onions. They can be stored in the refrigerator for about a week.
From Wikipedia: Scallions (green onion, spring onion and salad onion) are vegetables of various Allium onion species. Scallions have a milder taste than most onions. Their close relatives include the garlic, shallot, leek, chive, and Chinese onion. Although the bulbs of many Allium species are used as food, the defining characteristic of scallion species is that they lack a fully developed bulb. In common with all Allium species, scallions have hollow, tubular green leaves, growing directly from the bulb. These leaves are used as a vegetable; they are eaten either raw or cooked. The leaves are often chopped into other dishes, in the manner of onions or garlic. Also known as scallions or green onions, spring onions are in fact very young onions, harvested before the bulb has had a chance to swell.
The words scallion and shallot are related and can be traced back to the Greek ασκολόνιον ('askolonion') as described by the Greek writer Theophrastus. This name, in turn, seems to originate from the name of the ancient Canaan city of Ashkelon. The plant itself apparently came from farther east of Europe.
Species and cultivars of scallions include the following:
Scallions have various other common names throughout the world. These names include spring onion, green onion, table onion, salad onion, onion stick, long onion, baby onion, precious onion, yard onion, gibbon, syboe, or scally onion. Scallion and its many names can also be mistakenly used for the young plants of the shallot (A. cepa var. aggregatum, formerly A. ascalonicum), harvested before bulbs form, or sometimes when slight bulbing has occurred.
See the link above for an extensive list of common names used for scallions throughout the world.