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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

Floccose scales are thus left on the surface of the stem below the annulus, as in the left hand plant of Fig. 53.
Studies of American Fungi. Mushrooms, Edible, Poisonous, etc. George Francis Atkinson

The annulus is not persistent, and the gills are crowded and notched.
Studies of American Fungi. Mushrooms, Edible, Poisonous, etc. George Francis Atkinson

annulus auri cum sapphiro qui fuit fratris Willelmi quondam Dublin archiepiscopi defuncti.
Finger-Ring Lore William Jones

Armillate: with a ring or annulus of raised or different tissue.
Explanation of Terms Used in Entomology John. B. Smith

annulus, or ring, like that of the spore-case of most Ferns.
The Elements of Botany Asa Gray

Ring: a circle or annulus, usually margining a discolored spot.
Explanation of Terms Used in Entomology John. B. Smith

The veil is very handsome, and the way in which the annulus is formed from it is very interesting.
Studies of American Fungi. Mushrooms, Edible, Poisonous, etc. George Francis Atkinson

This was called the astronomical ring, annulus astronomicus.
Finger-Ring Lore William Jones

It is usually whitish near the upper end, but dull brown or reddish brown below the annulus, sometimes distinctly yellowish.
Studies of American Fungi. Mushrooms, Edible, Poisonous, etc. George Francis Atkinson

The sporangia have very short stalks, and are not provided with an annulus at all.
How to Know the Ferns S. Leonard Bastin

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.