annulus

American Heritage® Dictionary of the English Language, Fourth Edition

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

Century Dictionary and Cyclopedia

1. n. A ring-like space or area contained between the circumferences of two concentric circles.
2. n. In anatomy, a ring-like part, opening, etc.: used in Latin phrases.
3. n. (See below.) In botany: The elastic ring which surrounds the spore-case of most ferns.
4. n. In mosses, an elastic ring of cells lying between the lid and the base of the peristome or orifice of the capsule.
5. n. In fungi, the slender membrane surrounding the stem in some agarics after the cap has expanded.
6. n. 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.
7. n. In entomology, a narrow encircling band, generally of color; sometimes a raised ring.
8. n. 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.
9. n. In the Equisetaceæ, the sheath below the spike formed by the union of the bases of the leaves.
10. n. In diatoms, the rim of silex formed within the frustules of some genera.
11. n. The fleshy rim of the corolla in milkweeds.
12. n. One of the external subdivisions of the body of a leech, resembling a segment of the body of an earthworm. A single annulus, however, does not correspond to an internal segment. From 3 to 5 or even 12 annuli correspond to a segment in different genera.

Wiktionary

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

GNU Webster's 1913

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

WordNet 3.0

1. n. a toroidal shape
2. n. (Fungi) a remnant of the partial veil that in mature mushrooms surrounds the stem like a collar