Source : Webster's Revised Unabridged Dictionary (1913)

Annulus \An"nu*lus\, n.; pl. {Annuli}. [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. (Zo["o]l.) Ring-shaped structures or markings, found in,
      or upon, various animals.

Source : WordNet®

annulus
     n 1: a toroidal shape; "a ring of ships in the harbor"; "a halo
          of smoke" [syn: {ring}, {halo}, {anulus}, {doughnut}, {anchor
          ring}]
     2: (Fungi) remnant of the partial veil that in mature mushrooms
        surrounds the lower part of the stem
     [also: {annuli} (pl)]

annuli
     See {annulus}