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A geometric annulus (sense 2).

Learned borrowing from Medieval Latin ānnulus, a misspelling of Latin ānulus (ring, especially one worn on a finger), from ānus (ring) (from Proto-Indo-European *h₁eh₂no- (ring)) + -ulus (diminutive suffix).[1]

The plural form annuli is a learned borrowing from Medieval Latin ānnulī.



annulus (plural annuluses or annuli)

  1. A ring- or donut-shaped area or structure.
    Hyponym: torus
  2. (geometry) The region in a plane between two concentric circles of different radii.
  3. (topology) Any topological space homeomorphic to the region in a plane between two concentric circles of different radius.
    Synonym: cylinder
  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.

Derived terms[edit]

Related terms[edit]



  1. ^ Compare “annulus, n.”, in OED Online Paid subscription required, Oxford, Oxfordshire: Oxford University Press, September 2023; “annulus, n.”, in Lexico,; Oxford University Press, 2019–2022.

Further reading[edit]



From ānus (ring) +‎ -ulus.



ānnulus m (genitive ānnulī); second declension

  1. Alternative form of ānulus


Second-declension noun.

Case Singular Plural
Nominative ānnulus ānnulī
Genitive ānnulī ānnulōrum
Dative ānnulō ānnulīs
Accusative ānnulum ānnulōs
Ablative ānnulō ānnulīs
Vocative ānnule ānnulī



  • annulus in Charles du Fresne du Cange’s Glossarium Mediæ et Infimæ Latinitatis (augmented edition with additions by D. P. Carpenterius, Adelungius and others, edited by Léopold Favre, 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