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

Derived 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; see 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

Joe’s Kansas City has the Z-Man sandwich: brisket, provolone, onion ring and barbecue sauce on a brioche roll, mindblowing.

From The Wall Street Journal • Jun. 4, 2026

At their core, they’re built from ingredients that already know how to get along with vegetables: garlic, onion, oregano, parsley, acidity and a touch of sweetness.

From Salon • Jun. 2, 2026

Her lunch was bread with onion, salt and pickle.

From BBC • Jun. 1, 2026

To make, combine cottage cheese with garlic powder, onion powder, turmeric, ground cumin, black pepper and a pinch of salt.

From Salon • May 24, 2026

She, too, seemed to be misty.eyed all of sudden; perhaps it was a consequence of some errant fumes from the onion soup being served at another table.

From "The Hidden Gallery" by Maryrose Wood