annulus
n//-li//cirkelring
(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.
(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.