-iumsuff. Chemical element or group: californium.[New Latin -ium, neuter n. suff., from Greek -ion.]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.-ium or sometimes -umsuffix forming nouns1. (Chemistry) indicating a metallic element: platinum; barium. 2. (Chemistry) (in chemistry) indicating groups forming positive ions: ammonium chloride; hydroxonium ion. 3. (Biology) indicating a biological structure: syncytium. [New Latin, from Latin, from Greek -ion, diminutive suffix]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014-ium , a suffix found on nouns borrowed from Latin, esp. derivatives of verbs (odium; tedium; colloquium; delirium), deverbal compounds with the initial element denoting the object of the verb (nasturtium), other types of compounds (equilibrium; millennium), and derivatives of personal nouns, often denoting the associated status or office (collegium; consortium; magisterium). [