Welsh onions (Allium fistulosum L., Syn.: A. altaicum, . ceratophyllum, Cepa sissilis, C. ventricosa) are often called scallions. There are also many regional names including spring onions, green onions, table onions, salad onions, onion sticks, long onions, baby onions, precious onions, yard onions, gibbons, syboes, and scally onions.
From Wikipedia: “Scallion, green onion, and spring onion are English names (with many other regional names throughout the world) for various Allium species. All of the Allium have hollow green leaves (like the common onion), but these are used while they lack a fully developed root bulb. Used as a vegetable, they are eaten either raw or cooked. Scallions have a milder taste than most onions.”
“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.”
“Spring onions may be cooked or used raw as a part of salads, salsas, or Asian recipes. Diced scallions are used in soup, noodle and seafood dishes, as well as sandwiches, curries or as part of a stir fry. In many Eastern sauces, the bottom half-centimetre (quarter-inch) of scallions roots is commonly removed before use. ...
An oil, scallion oil is sometimes made from the green leaves. The leaves are chopped, lightly cooked, oil is added and then it is liquidised. The oil is then used as a garnish.”
“Species and cultivars which may be called "scallions" include:
Regional and other names:
“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.