-ium
/ɪəm/suffix
1Forming nouns adopted unchanged from Latin (such as alluvium) or based on Latin or Greek words (such as euphonium).
2 (also -um) Forming names of metallic elements: cadmium magnesium
3Denoting a region of the body: pericardium
4Denoting a biological structure: mycelium
Origin
Modern Latin in sense 2, sense 3, sense 4, via Latin from Greek -ion.