Annulus (mathematics)
In mathematics, an annulus (the Latin word for "little ring", with plural annuli) is a ring-shaped object, especially a region bounded by two concentric circles. The adjectival form is annular (as in annular eclipse).
The open annulus is topologically equivalent to both the open cylinder S × (0,1) and the punctured plane. Informally, it has the shape of a hardware washer.