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.

6. The membranous remnants of a partial veil which leaves a ring on the stem of a mushroom.

7. (oil and gas production) The space contained between the centre well bore and any external tubing. Sometimes used for separated gas flow.