Ium definitions

Chemical element or group.

Californium.

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Forming the names of certain biological structures.

Conidium, syncytium.

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Forming Modern Latin names for chemical elements.

Sodium.

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Forming the names of certain positive ions.

Ammonium, carbonium.

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Used to form the names of metal elements, after the style of early-named elements, as well as the isotopes of hydrogen.
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By extension, appended to common words to create scientific-sounding or humorous-sounding fictional substance names.
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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.
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Origin of ium

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