‖An″nu‐lus (�), n.; pl.Annuli (�). 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.