annulus Definition

annulus
noun annuli

1. geom.
The figure formed by two concentric circles on a plane surface, ie a disc with a central hole.
2. biol.
Any ring-shaped structure, eg the ring of cells that surrounds the spore-bearing structure in ferns, and by constriction causes the structure to rupture and release its spores.

Etymology: 16c, meaning ‘a ring or ring-shaped object': medieval Latin, from classical Latin anulus ring.