-ium Definition

ēəm, yəm
suffix
Chemical element or group.
Californium.
American Heritage
By extension, appended to common words to create scientific-sounding or humorous-sounding fictional substance names.
Wiktionary

Used to indicate the setting where a given activity is carried out: gymnasium, auditorium, stadium, colloquium, planetarium, podium, sanatorium. Words so formed often take "-a" for the plural.

Wiktionary
affix
Forming Modern Latin names for chemical elements.
Sodium.
Webster's New World
Forming the names of certain positive ions.
Ammonium, carbonium.
Webster's New World
Forming the names of certain biological structures.
Conidium, syncytium.
Webster's New World

Origin of -ium

  • From Latin -um (“neuter singular morphological suffix”), based on Latin terms for metals, such as ferrum.

    From Wiktionary

  • New Latin -ium neuter n. suff. from Greek -ion

    From American Heritage Dictionary of the English Language, 5th Edition

  • ModL < L, ending of certain neuter nouns

    From Webster's New World College Dictionary, 5th Edition