Noun

1. A ring- or donut-shaped area or structure.
2. The region between two concentric circles of different radius.
3. The ring of the sun not covered by the moon in an annular solar eclipse.
4. Structure in a fern that consists of differentially thick-walled cells on a sporangium that bend and distort as a result of drying.
5. The membranous remnants of a partial veil which leaves a ring on the stem of a mushroom.
6. The space contained between the centre well bore and any external tubing. Sometimes used for separated gas flow.