Allium fistulosum L. (Welsh onion, Japanese bunching onion) is a perennial onion. Other names that may be applied to this plant include green onion, spring onion, scallion, escallion, and salad onion. These names are ambiguous, as they may also be used to refer to any young green onion stalk, whether grown from Welsh onions, common onions, or other similar members of the genus Allium (see scallion). The species is very similar in taste and odor to the related common onion, Allium cepa, and hybrids between the two (tree onions) exist. The Welsh onion, however, does not develop bulbs, and possesses hollow leaves ("fistulosum" means "hollow") and scapes. Large varieties of the Welsh onion resemble the leek, such as the Japanese 'negi', whilst smaller varieties resemble chives. Many Welsh onions can multiply by forming perennial evergreen clumps. Next to culinary use, it is also grown in a bunch as an ornamental plant.

Historically, the Welsh onion was known as the cibol.

The name "Welsh onion" has become a misnomer in modern English, as Allium fistulosum is not indigenous to Wales. "Welsh" preserves the original meaning of the Old English word "welisc", or Old German "welsche", meaning "foreign" (compare wal- in "walnut", of the same etymological origin). The species originated in Asia, possibly Siberia or China. In Wales, the spring onion has a dialectal variation, jibbons or sibwns (pronounced 'shiboons') which originates from the French 'ciboule.'