What does annulus mean?
The definition of annulus
An”nu*lus, n.; pl. Annuli.
Etym: [L.]

1 A ring; a ringlike part or space.
2 (Geom.)

(a) A space contained between the circumferences of two circles, one within the other.
(b) The solid formed by a circle revolving around a line which is the plane of the circle but does not cut it.

3 (Zoöl.)
Ring-shaped structures or markings, found in, or upon, various animals.