annulus

noun

1. A ring- or donut-shaped area, object, or structure.

   “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— […]”

2. 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.
3. 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).
4. 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.
5. The membranous remnants of a partial veil which leaves a ring on the stem of a mushroom.
6. 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.
7. The region in a plane between two concentric circles of different radii.
8. Any topological space homeomorphic to the region in a plane between two concentric circles of different radii.
9. 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.

   “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.”