Definition:Annulus (Geometry)

Definition
An annulus is a plane figure whose boundary consists of $2$ concentric circles:


 * Annulus.png

In the above diagram, the annulus is the colored area.

Also known as
The more contemporary word ring can sometimes be seen for annulus.

However, as the term ring is ubiquitous in the field of abstract algebra to refer to an algebraic structure of a particular type, it is the policy of to use the term annulus instead, so as to reduce the possibility of confusion.