An annulus (from Latin anulus, "little ring") is a ring-shaped geometric figure, or more generally, a term used to name a ring-shaped object. The adjective form is annular (for example, an annular eclipse).
In complex analysis an annulus ann(a; r, R) in the complex plane is an open region defined by:
- r < | z - a | < R.
If r is 0, the region is known as the punctured disk of radius R around the point a.
The open annulus is topologically equivalent to the open cylinder 
.