onion

UK: /ˈʌnjən/ US: /ˈʌnjən/ ,(un′yən)

WordReference Random House Learner's Dictionary of American English © 2025

on•ion /ˈʌnyən/   n. [countable]

1. Plant Biology a plant of the amaryllis family, having a white or yellow, strong-smelling bulb that can be eaten.
2. Plant Biology this bulb, used in cooking.

WordReference Random House Unabridged Dictionary of American English © 2025

on•ion  (un′yən), n. 

1. Plant Biology a plant, Allium cepa, of the amaryllis family, having an edible, succulent, pungent bulb.
2. Plant Biology any of certain similar plants.
3. Plant Biology the bulb of the onion plant.
4. the flavor or odor of this bulb.
5. Slang Terms a person:
   He's a tough onion.
6. Idioms know one's onions, [Slang.] to know one's subject or business thoroughly;
   be capable or proficient.

adj. 

7. containing or cooked with onions:
   onion soup.
8. of, pertaining to, or resembling an onion.

• Latin ūniōn- (stem of ūniō) a unity, large pearl, onion; see union
• Old French oignon
• Middle English onyon 1325–75

on′ion•like′, adj. 
on′ion•y, adj. 

Collins Concise English Dictionary © HarperCollins Publishers::

onion / ˈʌnjən/ noun

1. an alliaceous plant, Allium cepa, having greenish-white flowers: cultivated for its rounded edible bulb
2. the bulb of this plant, consisting of concentric layers of white succulent leaf bases with a pungent odour and taste
3. know one's onions ⇒ to be fully acquainted with a subject

Etymology: 14th Century: via Anglo-Norman from Old French oignon, from Latin unio onion, related to union

ˈoniony adjective