Annulus
From Latin: annulus - "small ring"
Definition: The area between two coplanar concentric circles. A flat ring-shaped object.
Try this Drag either orange dot to resize the circles. The yellow area between them is an annulus.

Throwing ring in the shape of an annulus An annulus is a flat ring shaped object, much like the throw-ring shown on the right. One way to think of it is a circular disk with a circular hole in it. The outer and inner circles that define the ring are concentric (share a common center point).

The dimensions of an annulus are defined by the two radii R2, R1 in the figure above, which are the radii of the outer ring and the inner 'hole' respectively.

The area of the annulus can be found by subtracting the area of the 'hole' from the area of the overall disk. See Area of an Annulus

The adjective form is 'annular'. So for example the ring on the right could be called an 'annular plastic ring'.

Other circle topics

General

Equations of a circle

Angles in a circle

Arcs