A ringlike structure, or any body part that is shaped like a ring; which is applied to many small ring-shaped structures; such as, parts in a plant or animal, for example, a growth ring on fish scales.

The base of a heart valve that supports the valve's leaflets is called the annulus and a ring-shaped pancreas that encircles the duodenum is an annular pancreas.

Annulus is a misspelling of the Latin anulus, "a little ring", the diminutive of the Latin anus, "a ring of more substantial size"; so, keep in mind that anulus is now the preferred spelling for any "ringlike structure" reference.