a plant, Allium cepa, of the amaryllis family, having an edible, succulent, pungent bulb.
any of certain similar plants.
the bulb of the onion plant.
the flavor or odor of this bulb.
Slang. a person: He's a tough onion.
containing or cooked with onions: onion soup.
of, pertaining to, or resembling an onion.
know one's onions, Slang. to know one's subject or business thoroughly; be capable or proficient.

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

onionlike, adjective
oniony, adjective
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
onion (ˈʌnjən)
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 (Brit) know one's onions to be fully acquainted with a subject
[C14: via Anglo-Norman from Old French oignon, from Latin unio onion, related to union]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

1130, from Anglo-Fr. union, from O.Fr. oignon (formerly also oingnon), from L. unionem (nom. unio), colloquial rustic Roman for "a kind of onion," also "pearl," lit. "one, unity;" sense connection is the successive layers of an onion, in contrast with garlic or cloves. O.E. had ynne (in ynne-leac), from
the same L. source, which also produced Ir. inniun, Welsh wynwyn and similar words in Gmc. In Du., the ending in -n was mistaken for a plural inflection and new sing. ui formed. The usual I.E. name is represented by Gk. kromion, Ir. crem, Welsh craf, O.E. hramsa, Lith. kremuse. The usual L. word was cepa, a loan from an unknown language; cf. O.Fr. cive, O.E. cipe, and, via L.L. dim. cepulla, It. cipolla, Sp. cebolla, Pol. cebula. Ger zweibel also is from this source, but altered by folk etymology in O.H.G. (zwibolla) from words for "two" and "ball." Onion ring is attested from 1952. Onions, the surname, is attested from 1159 (Ennian), from O.Wesh Enniaun, ult. from L. Annianus, which was associated with Welsh einion "anvil."
Online Etymology Dictionary, © 2010 Douglas Harper
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Bible Dictionary

Onion definition

The Israelites in the wilderness longed for the "onions and garlick of Egypt" (Num. 11:5). This was the _betsel_ of the Hebrews, the Allium cepe of botanists, of which it is said that there are some thirty or forty species now growing in Palestine. The onion is "the 'undivided' leek, _unio_, _unus_, one."

Easton's 1897 Bible Dictionary
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