an alliaceous plant, Allium cepa, having greenish-white flowers: cultivated for its rounded edible bulb the bulb of this plant, consisting of concentric layers of white succulent leaf bases with a pungent odour and taste - know one's onions ⇒
to be fully acquainted with a subject
WordReference Random House Learner's Dictionary of American English © 2026
on•ion /ˈʌnyən/USA pronunciation
n. [countable]
WordReference Random House Unabridged Dictionary of American English © 2026- Plant Biologya plant of the amaryllis family, having a white or yellow, strong-smelling bulb that can be eaten.
- Plant Biologythis bulb, used in cooking.
on•ion
(un′yən),USA pronunciation n.
adj.
on′ion•like′, adj.
on′ion•y, adj.
- Plant Biologya plant, Allium cepa, of the amaryllis family, having an edible, succulent, pungent bulb.
- Plant Biologyany of certain similar plants.
- Plant Biologythe bulb of the onion plant.
- the flavor or odor of this bulb.
- Slang Termsa person:He's a tough onion.
- Idioms know one's onions, [Slang.]to know one's subject or business thoroughly;
be capable or proficient.
adj.
- containing or cooked with onions:onion soup.
- 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•y, adj.
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