Annulus

noun

1. A ring- or donut-shaped area or structure.
2. The region in a plane between two concentric circles of different radii.
3. Any topological space homeomorphic to the region in a plane between two concentric circles of different radius.
4. The ring of the sun not covered by the moon in an annular solar eclipse.
5. Structure in a fern that consists of differentially thick-walled cells on a sporangium that bend and distort as a result of drying.