Other
- Annulus (construction), outer gear ring in an epicyclic gearing
- Annulus (mathematics), geometric figure
- Annulus (botany), structure on fern sporangia
- Annulus (mycology), structure on mushroom
- Annulus (zoology), growth-induced feature
- Annulus (firestop), site of construction issue
- Annulus (oil well), void between concentric cylinders
- The Annulus, fictional organization in comics
- Annulation, type of ring-producing chemical reaction
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