-ium

suff.

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

-um

suffix forming nouns

1. (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).

[< New Latin, Latin]

-ium

a suffix of scientific coinages, occurring esp. in names of elements ( barium; titanium) and names of plant and fungus structures ( mycelium; pollinium), and in Latinizations of the Greek formative -ion (pericardium), as well as coinages modeled on such words ( epithelium; periodontium).