-ellum

Forming nouns.

Latin diminutive suffix ‑ellum.

This occurs mainly in technical terms for parts of animals or plants: the cerebellum (Latin, diminutive of cerebrum, brain) is a part of the brain at the back of the skull in vertebrates; a flagellum (Latin, diminutive of flagrum, scourge) is a microscopic whip-like appendage which enables many single-celled animals to swim; a cribellum (Latin, diminutive of cribrum, sieve) is an additional spinning organ in some spiders; a labellum (Latin, diminutive of labrum, lip) is one of a pair of lobes at the tip of the proboscis in some insects, or a central petal at the base of an orchid flower. These all have plurals in ‑a (see ‑a²): labella, flagella.

However, antebellum, occurring or existing before a particular war, especially the US Civil War, and the equivalent post-bellum, both derive from Latin bellum, war, which is not a diminutive; vellum, fine parchment made originally from the skin of a calf, comes from Old French velin.