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definitions

-onium

Chemistry.
  1. a suffix used in the names of complex cations: oxonium.

Origin of -onium

extracted from ammonium
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
-onium in Medicine

-onium

suff.
  1. Containing a positively charged radical:carbonium.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.