Annulus \An"nu*lus\, n.; pl. {Annuli}. [L.]
1. A ring; a ringlike part or space.
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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.
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3. (Zool.) Ring-shaped structures or markings, found in, or
upon, various animals.
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