An annulus is the shape made by the space between two concentric circles.
Concentric means that both circles are drawn from the SAME centre.
In the diagram, the inner diameter is shown by the dimension 'd'; the outer diameter by the dimension'D'; and the width by 'w'.
The plural of annulus is annuli
In real life it is the shape of the washer used underneath a nut when screwed onto a bolt. It is also commonly seen in garden layouts or in the design of a flower-border around some ornamental feature, such as a fountain or statue. Much geometrical pattern-work uses the annulus.
It is sometimes called a ring.