Forming nouns; chemical elements.
[Via Latin from Greek -ion.]
This ending appears in a wide range of words, often in technical and scientific use, some of which are unchanged from their Latin forms: alluvium, colloquium, delirium, geranium, odium, proscenium.
It has also been used to form the names of most of the chemical elements discovered or isolated since the beginning of the nineteenth century, when Sir Humphry Davy (1778–1829) used it for sodium, potassium, magnesium, and aluminium (US aluminum, based on the older Latinate form -um); some others are cadmium, iridium, lithium, osmium, palladium, rhodium, titanium, and uranium. The ending has also been used for various cationic forms (able to take the role of metals in reactions), such as ammonium.
A further set marks names for parts of the body, commonly derived via modern Latin from Greek precursors: cranium (Greek kranion, skull); epithelium (Greek epi, above, plus thēlē, teat), the thin tissue forming the outer layer of a body's surface; ilium (Latin, singular of ilia, flanks), the large broad bone forming the upper part of each half of the pelvis; pericardium (Greek peri, around, plus kardia, heart), the membrane enclosing the heart.
Others are names for botanical and biological structures: archegonium (Greek arkhegonos, founder of a race), the female sex organ in mosses, liverworts, ferns, and most conifers; ommatidium (Greek omma, ommat-, eye), each of the optical units that make up the compound eye of an insect; uropygium (Greek orros, sacral bone, plus pugē, rump), the rump of a bird, supporting the tail feathers. See also -orium.