молодой зеленый лук, у которого еще не образовалась белая "репка". Иногда под scallion имею в виду лук-порей, но не часто.

The common name scallion is associated with various members of the genus Allium that lack a fully-developed bulb. They tend to be milder tasting than other onions and are typically used raw in salads.
en.wikipedia.org/wiki/Scallion

Also called green onions, scallions have a milder flavor than onions and are used in salads, soups and many other dishes for flavor.
wgby.org/localprograms/onthemenu/pages/recipes/glossary.html