(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 cavity or vessel, particularly referring to the sinoatrial node of the heart and its role in 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.