annulus

noun, plural an·nu·li [an-yuh-lahy] /ˈæn yəˌlaɪ/, an·nu·lus·es.

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

noun plural -li (-ˌlaɪ) or -luses

1. the area between two concentric circles
2. a ring-shaped part, figure, or space

n. pl. an•nu•lus•es

1. A circular or ring-shaped structure.

Plural annuluses annuli (ăn′yə-lī′)

1. A ringlike figure, part, structure, or marking, such as a growth ring on the scale of a fish.
2. 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.
3. 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.
4. The figure bounded by and containing the area between two concentric circles.