sinus

noun

• 1 (often sinuses) Anatomy & Zoology a cavity within a bone or other tissue, especially one in the bones of the face or skull connecting with the nasal cavities.
• an irregular venous or lymphatic cavity, reservoir, or dilated vessel.
• Medicine an infected tract leading from a deep-seated infection and discharging pus to the surface.
• Botany a rounded notch between two lobes on the margin of a leaf or petal.

• 2 [as modifier] Physiology relating to or denoting the sino-atrial node of the heart or its function of regulating the heartbeat:sinus rhythm sinus tachycardia