-ium

plural -iums

1. uncountableusuallychemistryUsed to form the names of metal elements, after the style of early-named elements, as well as the isotopes of hydrogen.
2. uncountableusuallychemistryUsed to form the temporary systematic element name of a metallic or nonmetallic element which is postulated to exist, or which has been newly synthesized and has not yet been assigned a permanent name.
3. uncountableusuallychemistryUsed to form the name of polyatomic cations. Common examples are quaternary ammonium compounds used as neuromuscular blocking agents, cholinergic agents, anticholinergic agents, antibacterials/antiseptics, or other agents.

   Alternative form: -onium

4. broadlyhumorousuncountableusuallyAppended to common words to create scientific-sounding or humorous-sounding fictional substance names.

   Stupidium: Exposure to this Element can cause very silly behavior.

   2007, Jason Lethcoe, Wishing Well, appendix, page ii

   2 more examples

   According to the FAQ, a site's coolness can be attributed to a trace element called coolium.

   1997, Bryan Pfaffenberger, Official Microsoft Internet Explorer 4 book, page 34

   We're at the North Pole, aren't we? Let's just call the new element Santa Clausium!

   2009, Selena Kitt, Quickies, page 91

plural -iums · plural -ia

1. Used to form the name of an aggregation or mass of something, such as biological tissue

   epithelium, pollinium

2. Forms the name of biological structures and parts.

   clinanthium, periodontium

plural -iums · plural -ia

1. Used to indicate the setting where a given activity is carried out

   auditorium, gymnasium, planetarium