■ suffix
1》 forming 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.
3》 denoting a region of the body: pericardium.
4》 denoting a biological structure: mycelium.
Origin
mod. L. in senses 2, 3, and 4, via L. from Gk -ion .