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.
2. A ring or group of thick-walled cells around the sporangia of many ferns that functions in spore release.
The ringlike remains of a broken partial veil, found around the stipes of certain mushrooms.3. In mathematics, the figure bounded by and containing the area between two concentric circles.