al·li·um (?l??-?m)n. Any of numerous, usually bulbous plants of the genus Allium, having long stalks bearing clusters of variously colored flowers and including many ornamental and food plants, such as onions, leeks, chives, garlic, and shallots.[New Latin Allium, genus name, from Latin allium, garlic, variant of earlier ?lium, probably of Mediterranean substrate origin.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.allium (?æl??m) n (Plants) any plant of the genus Allium, such as the onion, garlic, shallot, leek, or chive: family Alliaceae[C19: from Latin: garlic]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014al?li?um (?æl i ?m) n. any bulbous plant of the genus Allium, of the amaryllis family, having flowers in a round cluster, including the onion, leek, shallot, garlic, and chive. [1800?10;