Annulus (mathematics)

In mathematics, an annulus (pl.: annuli or annuluses) is the region between two concentric circles. Informally, it is shaped like a ring or a hardware washer. The word "annulus" is borrowed from the Latin word anulus or annulus meaning 'little ring'. The adjectival form is annular (as in annular eclipse).

The open annulus is topologically equivalent to both the open cylinder S¹ × (0,1) and the punctured plane.