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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.



  • (ring-shaped structure): torus

Derived terms[edit]





ānnulus m (genitive ānnulī); second declension

  1. Alternative form of ānulus


Second declension.

Case Singular Plural
nominative ānnulus ānnulī
genitive ānnulī ānnulōrum
dative ānnulō ānnulīs
accusative ānnulum ānnulōs
ablative ānnulō ānnulīs
vocative ānnule ānnulī


  • du Cange, Charles (1883), “annulus”, in G. A. Louis Henschel, Pierre Carpentier, Léopold Favre, editors, Glossarium Mediæ et Infimæ Latinitatis (in Latin), Niort: L. Favre
  • 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