Annulus: A ringlike structure, or any body part that is shaped like a ring. Applied to many small ring-shaped structures.
The base of a heart valve that supports the valve's leaflets is called the annulus. 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).