A ring- or donut-shaped area or structure.
(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).
A structure surrounding a sporangium in botany or the membranous remnants of a partial veil in mycology.
A region in a plane between two concentric circles of different radii, or any topological space homeomorphic to this region.
(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.