The Collaborative International Dictionary of English v.0.48:

Annulus \An"nu*lus\, n.; pl. Annuli. [L.]
   1. A ring; a ringlike part or space.
      [1913 Webster]

   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.
          [1913 Webster]

   3. (Zool.) Ring-shaped structures or markings, found in, or
      upon, various animals.
      [1913 Webster]

WordNet (r) 3.0 (2006):

annulus
    n 1: a toroidal shape; "a ring of ships in the harbor"; "a halo
         of smoke" [syn: ring, halo, annulus, doughnut,
         anchor ring]
    2: (Fungi) a remnant of the partial veil that in mature
       mushrooms surrounds the stem like a collar [syn: annulus,
       skirt]