Annulus
From Latin: annulus  "small ring"
Try this Drag either orange dot to resize the circles. The yellow area between them is an annulus.
An annulus is a flat ring shaped object, much like the throwring 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
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