# annulus
## English
### Etymology

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

### Pronunciation

* Singular:
** (Received Pronunciation) IPA: /ˈænjʊləs/
** (General American) IPA: /ˈænjələs/
** Audio (US):(file)
** Hyphenation: an‧nu‧lus
* Plural (annuli):
** (Received Pronunciation) IPA: /ˈænjʊlaɪ/, /-li/
** (General American) IPA: /ˈænjəlaɪ/, /-li/
** Hyphenation: an‧nu‧li

### Noun

annulus (plural annuluses or annuli)
* A ring- or donut-shaped area or structure.
    Hyponym: torus
** 1761, [Laurence Sterne], chapter IX, in The Life and Opinions of Tristram Shandy, Gentleman, volume III, London: […] R[obert] and J[ames] Dodsley […], →OCLC, pages 29–30:But by the knots I am ſpeaking of, may it pleaſe your reverences to believe, that I mean good, honeſt, deviliſh tight, hard knots, made bona fide, as Obadiah made his;—in vvhich there is no quibbling proviſion made by the duplication and return of the tvvo ends of the ſtrings through the annulus or nooſe made by the ſecond implication of them— […]
* (anatomy) A ring of fibrous tissue; specifically (cardiology), such a ring around an opening of a heart valve, to which the valve leaflets and muscle fibres of the atria and ventricles are attached; an annulus fibrosus cordis.
* (astronomy) A ring of light in a celestial body, especially when caused by an annular eclipse (for example, when the Sun and Moon are in line with the Earth, but the Moon does not completely cover the Sun's disc).
* (biology)
** (botany) A structure surrounding a sporangium (or part of it) which shrinks and causes it to rupture for spore dispersal; specifically, in a fern: a structure around about two-thirds of the sporangium consisting of differentially thick-walled cells which dry and distort the sporangium; and in a moss: a complete ring of cells around the tip of the sporangium which dissolves to cause the tip to detach.
** (mycology) The membranous remnants of a partial veil which leaves a ring on the stem of a mushroom.
** (ichthyology) A dark ring on a fish's scale that is formed when a fish's growth rate slows down in the winter due to low food intake and the scale's circuli move closer to one another. The dark ring is used to estimate the fish's age, approximately one year per annulus.
    Coordinate terms: circulus, focus
* (mathematics)
** (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 radii.
    Synonym: cylinder
* (technology) In a well such as an oil well or water well: the space between a pipe or tube and any pipe, tube, casing, or sides of a hole surrounding it.
** 1950 September, “Notes and News: Pneumatic Buffer Stop, E.R.”, in The Railway Magazine, London: Tothill Press, →ISSN, →OCLC, page 642:Pressure balance is obtained by the air pressure in the cylinder operating on a supplementary piston in the buffer piston head, transmitting pressure to a small quantity of oil which is ported to an annulus between the buffer piston seals and the cylinder wall, so that the seal is always under opposing pressures; oil on one side and air on the other.

#### Derived terms

* annulotomy
* annulus fibrosus
* annulus of Zinn
* apical annulus
* attached annulus
* double annulus
* periannulus
* single annulus
* square annulus

#### Related terms

* annular
* annularity
* annularly
* annulary (archaic)
* annulate
* annulated
* annulation
* annule
* annulet
* exannulate
* mitral annulus
* quadriannulate
* sexannulate
* triannulate, triannulated

#### Translations
### References

* ↑ Compare “annulus, n.”, in OED Online ⁠, Oxford: Oxford University Press, September 2023; “annulus, n.”, in Lexico, Dictionary.com; Oxford University Press, 2019–2022.

### Further reading

* annulus (botany) on Wikipedia.Wikipedia
* annulus (mathematics) on Wikipedia.Wikipedia
* annulus (mycology) on Wikipedia.Wikipedia
* annulus (disambiguation) on Wikipedia.Wikipedia

## Latin
### Etymology

From ānus (“ring”) +‎ -ulus.

### Pronunciation

* (Classical Latin) IPA: /ˈaːn.nu.lus/, [ˈäːnːʊɫ̪ʊs̠]
* (modern Italianate Ecclesiastical) IPA: /ˈan.nu.lus/, [ˈänːulus]

### Noun

ānnulus m (genitive ānnulī); second declension
* Alternative form of ānulus

#### Declension

Second-declension noun.

#### Descendants

* English: annulus, annulet

### References

* "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