a suffix found on nouns borrowed from Latin, especially 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 ); -ium, also occurs in scientific coinages on a Latin model, as in names of metallic elements (barium; titanium ) and as a Latinization of Gk -ion (pericardium ).

< Neo-Latin, Latin, neuter suffix

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World English Dictionary
-ium or (sometimes) -um
suffix forming nouns
1.  indicating a metallic element: platinum; barium
2.  (in chemistry) indicating groups forming positive ions: ammonium chloride; hydroxonium ion
3.  indicating a biological structure: syncytium
[New Latin, from Latin, from Greek -ion, diminutive suffix]
-um or (sometimes) -um
suffix forming nouns
[New Latin, from Latin, from Greek -ion, diminutive suffix]

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American Heritage
Medical Dictionary

-ium suff.
Chemical element or group: californium.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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