n (topology) Any topological space homeomorphic to the region in a plane between two concentric circles of different radius.
n (astronomy) The ring of the sun not covered by the moon in an annular eclipse|annular solar eclipse.
n (botany) Structure in a fern that consists of differentially thick-walled cells on a sporangium that bend and distort as a result of drying.
n (mycology) The membranous remnants of a partial veil which leaves a ring on the stem of a mushroom.
n (context|oil and gas production) The space contained between the centre well bore and any external tubing. Sometimes used for separated gas flow.