An annulus

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 .