annulus

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A ringlike figure, part, structure, or marking, such as a growth ring on the scale of a fish.
• noun A ring or group of thick-walled cells around the sporangia of many ferns that functions in spore release.
• noun The ringlike remains of a broken partial veil, found around the stipes of certain mushrooms.
• noun Mathematics The figure bounded by and containing the area between two concentric circles.

from The Century Dictionary.

• noun In the Equisetaceæ, the sheath below the spike formed by the union of the bases of the leaves.
• noun In diatoms, the rim of silex formed within the frustules of some genera.
• noun The fleshy rim of the corolla in milkweeds.
• noun One of the external subdivisions of the body of a leech, resembling a segment of the body of an earthworm.
• noun A ring-like space or area contained between the circumferences of two concentric circles.
• noun In anatomy, a ring-like part, opening, etc.: used in Latin phrases.
• noun (See below.) In botany: The elastic ring which surrounds the spore-case of most ferns.
• noun In mosses, an elastic ring of cells lying between the lid and the base of the peristome or orifice of the capsule.
• noun In fungi, the slender membrane surrounding the stem in some agarics after the cap has expanded.
• noun In zoology: A thin chitinous ring which encircles the mantle in the Tetrabranchiata, connecting chitinous patches of the mantle into which the shell-muscles are inserted.
• noun In entomology, a narrow encircling band, generally of color; sometimes a raised ring.
• noun In astronomy, the ring of light seen about the edge of the moon in an annular eclipse of the sun. See annular eclipse, under annular.

from the GNU version of the Collaborative International Dictionary of English.

• noun A ring; a ringlike part or space.
• noun A space contained between the circumferences of two circles, one within the other.
• noun The solid formed by a circle revolving around a line which is the plane of the circle but does not cut it.
• noun (Zoöl.) Ring-shaped structures or markings, found in, or upon, various animals.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A ring- or donut-shaped area or structure.
• noun geometry The region in a plane between two concentric circles of different radius.
• noun topology Any topological space homeomorphic to the region in a plane between two concentric circles of different radius.
• noun astronomy The ring of the sun not covered by the moon in an annular solar eclipse.
• noun botany Structure in a fern that consists of differentially thick-walled cells on a sporangium that bend and distort as a result of drying.
• noun mycology The membranous remnants of a partial veil which leaves a ring on the stem of a mushroom.
• noun oil and gas production The space contained between the centre well bore and any external tubing. Sometimes used for separated gas flow.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a toroidal shape
• noun (Fungi) a remnant of the partial veil that in mature mushrooms surrounds the stem like a collar