# Annulus (mathematics)

## Region between two concentric circles / From Wikipedia, the free encyclopedia

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In mathematics, an **annulus** (plural **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 *S*^{1} × (0,1) and the punctured plane.