Allium fistulosum L., widely known as the Welsh onion, is a member of the onion family, Alliaceae. The species is very similar in taste and odor to the related garden onion, Allium cepa, and hybrids between the two (tree onions) exist. The Welsh onion, however, does not develop bulbs, and possesses hollow leaves and scapes (fistulosum means "hollow"). Large varieties of the Welsh onion resemble the leek, such as the Japanese 'negi', whilst smaller varieties resemble chives.

Besides Welsh onion, Allium fistulosum is known as 'scallion', 'green onion', 'spring onion' and 'Japanese bunching onion'. It is known in French as 'ciboule', and in Portuguese as cebolinha or cozida. Grubben, G.J.H. & Denton, O.A. (2004) Plant Resources of Tropical Africa 2. Vegetables. PROTA Foundation, Wageningen; Backhuys, Leiden; CTA, Wageningen. Historically, the Welsh onion was known as the 'cibol'.

The name 'Welsh onion' is 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". The species originated in Asia, possibly Siberia or China. Welsh onions are known as 蔥 (simplified:葱) (pinyin: cōng) in Chinese, 葱 or ネギ in Japanese (the Japanese transliteration, 'negi', is another term for Welsh onions), and 파 ('pa') in Korean.