1. [noun] A ring- or donut-shaped area or structure.
2. [noun] (geometry) The region in a plane between two concentric circles of different radius.
3. [noun] (topology) Any topological space homeomorphic to the region in a plane between two concentric circles of different radius.
4. [noun] (astronomy) The ring of the sun not covered by the moon in an annular solar eclipse.
5. [noun] (botany) Structure in a fern that consists of differentially thick-walled cells on a sporangium that bend and distort as a result of drying.
6. [noun] (mycology) The membranous remnants of a partial veil which leaves a ring on the stem of a mushroom.
7. [noun] (oil and gas production) The space contained between the centre well bore and any external tubing. Sometimes used for separated gas flow.
8. [noun] A toroidal shape; 'a ring of ships in the harbor'; 'a halo of smoke'.
9. [noun] (Fungi) a remnant of the partial veil that in mature mushrooms surrounds the stem like a collar.
