Definition from Wiktionary, the free dictionary
- A ring- or donut-shaped area or structure.
- (geometry) The region in a plane between two concentric circles of different radius.
- (topology) Any topological space homeomorphic to the region in a plane between two concentric circles of different radius.
- (astronomy) The ring of the sun not covered by the moon in an annular solar eclipse.
- (botany) Structure in a fern that consists of differentially thick-walled cells on a sporangium that bend and distort as a result of drying.
- (mycology) The membranous remnants of a partial veil which leaves a ring on the stem of a mushroom.
- (oil and gas production) The space contained between the centre well bore and any external tubing. Sometimes used for separated gas flow.
- (topological space): cylinder
- (ring-shaped structure): torus
Derived terms 
the region in a plane between two concentric circles of different radius
astronomy: the ring of the sun not covered by the moon
- Alternative form of ānulus.