annulus

A ring- or donut-shaped area or structure.

(geometry) The region in a plane between two concentric circles of different radii.

(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.