onion

American  

[uhn-yuhn] / ˈʌn yən /

noun

1. a plant, Allium cepa, of the amaryllis family, having an edible, succulent, pungent bulb.

2. any of certain similar plants.

3. the bulb of the onion plant.

4. the flavor or odor of this bulb.

5. Slang. a person.

   He's a tough onion.

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adjective

1. containing or cooked with onions.

   onion soup.

2. of, relating to, or resembling an onion.

idioms

1. know one's onions, to know one's subject or business thoroughly; be capable or proficient.

onion British  

/ 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. any of several related plants similar to A. cepa, such as A. fistulosum (Welsh onion)

4. slang to be fully acquainted with a subject

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onion More Idioms  

1. see know one's stuff (onions).

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Other Word Forms

• onionlike adjective
• oniony adjective

Etymology

Origin of onion

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

Explanation

An onion is a plant that grows as a bulb underground and can be harvested and eaten. Note: onions may be tasty, but they sure can make your breath stink. Onions are bulbs, like tulips or daffodils — most of the plant is hidden in the soil, with roots stretching down and stalks that grow upward and eventually bloom with small flowers. Onions are typically harvested in the fall, and they come in many shapes, sizes, and colors, all of which can be eaten. The Latin root of onion, unionem, was used in colloquial Roman to mean both "a type of onion" and "a pearl."

Example Sentences

They order $18 mezcal cocktails infused with shiitake mushrooms, red onion and jalapeño herb oil.

From The Wall Street Journal • Apr. 9, 2026

Add onion to the skillet and cook until fully softened and lightly golden, 4–5 minutes.

From Salon • Apr. 7, 2026

Quick pickled cucumbers, dressed with rice vinegar, sugar and sesame oil, bring texture and acidity while a classic Mexican mix of minced onion and cilantro finishes things off.

From The Wall Street Journal • Apr. 6, 2026

“The irony of it all is that when you look through all of the layers of the onion, there’s not really much there there,” he said.

From The Wall Street Journal • Mar. 13, 2026

Clive Pike's breath smells of cheese 'n' onion Outer Spacers, but who cares?

From "Black Swan Green" by David Mitchell