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a ring; a ringlike part, band, or space.
Geometry. the space between two concentric circles on a plane.
the veil remnant on a mushroom stalk.
a growth ring, as on the cross section of a tree trunk, that can be used to estimate age.
Historical Examples

This corresponds to the usual presence (in the Rhynchobdellidae) of three annuli to each segment.
Encyclopaedia Britannica, 11th Edition, Volume 5, Slice 7 Various

But in that group (Cystobranchus) there may be as many as eight annuli.
Encyclopaedia Britannica, 11th Edition, Volume 5, Slice 7 Various

Abdomen black, consisting of seven annuli or rings, besides the tail part, the last of which is edged with orange.
Illustrations of Exotic Entomology, Volume 2 Dru Drury

This phrase is somewhat obscure: “annuli ferrei sicut solebat fieri in cista Norensi.”
The Chronicle of Jocelin of Brakelond: A Picture of Monastic Life in the Days of Abbot Samson Jocelin de Brakelond

An eye-like spot in the Wings of many Lepidoptera, consisting of annuli of different colours, inclosing a central spot or pupil.
An Introduction to Entomology: Vol. IV (of 4) William Kirby

As remarked, the prevalent number of annuli to a segment is three in the Rhynchobdellidae.
Encyclopaedia Britannica, 11th Edition, Volume 5, Slice 7 Various

noun (pl) -li (-ˌlaɪ), -luses
the area between two concentric circles
a ring-shaped part, figure, or space
noun

See tree ring
n.

1560s, medical, from misspelling of Latin anulus “little ring, finger ring,” a diminutive of anus (see anus).

annulus an·nu·lus or an·u·lus (ān’yə-ləs)
n. pl. an·nu·lus·es or an·nu·li (-lī’)
A circular or ring-shaped structure.
annulus
(ān’yə-ləs)
Plural annuluses or annuli (ān’yə-lī’)

A ringlike figure, part, structure, or marking, such as a growth ring on the scale of a fish.

A ring or group of specialized cells around the sporangia of many ferns. By changing shape in response to variations in humidity, it breaks open the sporangium and then releases the spores with a whipping motion.

The ringlike remains of a membrane (called a veil), found around the stipes of certain basidiomycete mushrooms. The presence or absence of an annulus is often used to identify the species of an individual mushroom.

The figure bounded by and containing the area between two concentric circles.