noun

• (anatomy, zootomy) A pouch or cavity in a bone or other tissue, especially one in the bones of the face or skull connecting with the nasal cavities (the paranasal sinus).
• (anatomy) An irregular venous or lymphatic cavity, reservoir, or dilated vessel.
• (physiology, attributive) Relating to or denoting the sinoatrial node of the heart or its function of regulating the heartbeat.
• (pathology) An abnormal cavity or passage such as a fistula, leading from a deep-seated infection and discharging pus to the surface.
• (botany) A rounded notch or depression between two lobes or teeth in the margin of a leaf or petal.
• (geography) A bay of the sea; a recess in the shore.
• (trigonometry) Synonym of sine.