Annulus, being the Latin for 'ring' is a term used to describe various ring-shaped objects:

• Annulus (heart), the ring-like structure on a heart valve where the valve leaflets are anchored.
• Annulus (spine), the outer wall of the invertebral disc.
• Annulus (entomology), an antennal unit in simple antennae, or a ring-like marking or structure surrounding a joint or segment.
• Annulus (ichthyology), the annual rings that form in fish scales or bone sections.
• Annulus (mycology), the "ring" on a mushroom stem.
• Annulus (astronomy), the ring of light from the visible part of the photosphere seen at the greatest extent of an annular eclipse.
• Annulus (mathematics), a ring-shaped geometric figure.
• Annulus (oil well), the tube shaped void between the drill string and the formation when drilling an oil well.
• More generally, any ring shaped object, structure, or pattern.