annulus

Borrowed from Latin annulus.

annulus (plural annuluses or annuli)

1. A ring- or donut-shaped area or structure.
2. (geometry) The region in a plane between two concentric circles of different radius.
3. (topology) Any topological space homeomorphic to the region in a plane between two concentric circles of different radius.
4. (astronomy) The ring of the sun not covered by the moon in an annular solar eclipse.
5. (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. (mycology) The membranous remnants of a partial veil which leaves a ring on the stem of a mushroom.
7. (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

• annular

• annulate
• annulation

• (Classical) IPA^(key): /ˈaːn.nu.lus/, [ˈaːn.nʊ.ɫʊs]

ānnulus m (genitive ānnulī); second declension

1. Alternative form of ānulus

Second declension.

• annulus in Charles du Fresne du Cange’s Glossarium Mediæ et Infimæ Latinitatis (augmented edition, 1883–1887)
• annulus in Harry Thurston Peck, editor (1898) Harper's Dictionary of Classical Antiquities, New York: Harper & Brothers
• annulus in William Smith et al., editor (1890) A Dictionary of Greek and Roman Antiquities, London: William Wayte. G. E. Marindin