noun 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).
noun An irregular venous or lymphatic cavity, reservoir, or dilated vessel.
noun Relating to or denoting the sinoatrial node of the heart or its function of regulating the heartbeat.
noun An abnormal cavity or passage such as a fistula, leading from a deep-seated infection and discharging pus to the surface.
noun A rounded notch or depression between two lobes or teeth in the margin of a leaf or petal.