annulus

noun

1. 1

   A ring- or donut-shaped area or structure.

2. 2

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

3. 3

   Any topological space homeomorphic to the region in a plane between two concentric circles of different radius.

4. 4

   The ring of the sun not covered by the moon in an annular solar eclipse.

5. 5

   Structure in a fern that consists of differentially thick-walled cells on a sporangium that bend and distort as a result of drying.

6. 6

   The membranous remnants of a partial veil which leaves a ring on the stem of a mushroom.

7. 7

   (oil and gas production) The space contained between the centre well bore and any external tubing. Sometimes used for separated gas flow.