Sinus

noun

1. A pouch or cavity in any organ or tissue, especially the paranasal sinus.
2. A channel for transmitting venous blood.
3. A notch or depression between two lobes or teeth in the margin of an organ.
4. An abnormal cavity or passage such as a fistula, caused by the destruction of tissue.
5. A bay of the sea; a recess in the shore.