Suffix
-ium
Used to form the names of metal elements, after the style of early-named elements, as well as the isotopes of hydrogen.
By extension, appended to common words to create scientific-sounding or humorous-sounding fictional substance names.
Used to form the name of a mass (aggregation) of something, such as biologic tissue (for example, epithelium, pollinium). Words so formed often take "-a" for the plural.
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.
参考:https://en.wiktionary.org/wiki/-ium