(sī′nŭs)
pl. sinusessinus [L., sinus, curve, hollow]

1. A recess.
2. A cavity with a narrow opening.
3. An endothelium-lined, air-filled cavity within a bone. When lay people speak of sinuses, they usually mean the paranasal sinuses.
4. An endothelium-lined channel for venous blood.
5. A channel leading to an abscess.