What does annulus mean?
An annulus is a ring-shaped structure — formally, the region lying between two concentric circles, though the term extends to any object or marking shaped like a ring. It is a workhorse of technical vocabulary: geometers use it for the space between circles, anatomists for the tough outer band of intervertebral discs, botanists for the springy ring that flings spores from a fern, and mycologists for the skirt-like remnant on a mushroom's stalk. Growth rings on trees and fish scales are also described as annuli, which makes the word central to fields like dendrochronology. Derived from the Latin diminutive 'annulus', meaning 'little ring' (itself from 'annus', year), it carries a precise, scholarly tone and rarely appears in casual speech. Its adjective form is 'annular', familiar from phrases like 'annular eclipse', where the sun forms a glowing ring around the moon. The standard plural is 'annuli'.
nounA ring-shaped object, structure, or region; especially the area between two concentric circles or the ring-like marking found on fish scales and tree stumps.
- The space between two concentric circles; any ring-shaped region or object.
- In botany, the elastic ring of cells that opens a fern's sporangium.
- In mycology, the ring of tissue left on a mushroom's stem by a ruptured veil.
- A growth ring visible in fish scales, tree trunks, or similar structures.
"The cross-section revealed an annulus of darker wood marking each year's growth."
"Mathematicians calculate the area of the annulus by subtracting the inner circle's area from that of the outer circle."
"Under the microscope, each scale showed a clearly defined annulus corresponding to one year of growth."
The Latinate plural 'annuli' is standard in scientific writing; 'annuluses' is occasionally seen but less conventional.
"Counting the annuli on the otolith allowed researchers to estimate the fish's age."
Every time you count the rings of a tree to find its age, you're reading its annuli — nature's own diary written one circle at a time.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of annulus
Annulus comes directly from Latin, where it meant 'little ring' — a diminutive of 'annus', meaning 'year' or 'ring' — and English borrowed it as a learned term in the mid-sixteenth century. The same Latin root gave English a whole family of words tied to yearly cycles, including 'annual', 'annuity', and 'perennial', all ultimately reflecting the ancient sense of a cycle returning to its start. Closely related derivative forms include 'annular' (adjective) and 'annulate' (ring-bearing), both common in scientific description.
Related word forms
How annulus is actually used
A technical term used chiefly in mathematics, anatomy, botany, mycology, and engineering. In anatomy it appears most prominently in 'anulus fibrosus', the outer ring of an intervertebral disc (often spelled 'anulus' in anatomical nomenclature). Plural is usually the Latinate 'annuli'.
Easily confused with annulus
An 'annual' is something that occurs yearly, while an 'annulus' is a physical ring-shaped structure.
'Annular' is the adjective describing something ring-shaped, whereas 'annulus' is the noun naming the ring itself.