In mathematics, an annulus (the Latin word for "little ring" is anulus, with plural anuli) is a ring-shaped object, especially a region bounded by two concentric circles. The adjectival form is annular (as in annular eclipse).
The area of an annulus is the difference in the areas of the larger circle of radius R and the smaller one of radius r:
The area of an annulus can be obtained from the length of the longest interval that can lie completely inside the annulus, 2*d in the accompanying diagram. This can be proven by the Pythagorean theorem; the interval of greatest length that can lie completely inside the annulus will be tangent to the smaller circle and form a right angle with its radius at that point. Therefore, d and r are the sides of a right-angled triangle with hypotenuse R and the area is given by
The area of an annulus sector of angle θ, with θ measured in radians, is given by
If r is 0, the region is known as the punctured disk of radius R around the point a.
As a subset of the complex plane, an annulus can be considered as a Riemann surface. The complex structure of an annulus depends only on the ratio r/R. Each annulus ann(a; r, R) can be holomorphically mapped to a standard one centered at the origin and with outer radius 1 by the map
The inner radius is then r/R < 1.
The Hadamard three-circle theorem is a statement about the maximum value a holomorphic function may take inside an annulus.