A sinus is a sack or cavity in any organ or tissue, or an abnormal cavity or passage caused by the destruction of tissue. In common usage, "sinus" usually refers to the paranasal sinuses, which are air cavities in the cranial bones, especially those near the nose and connecting to it.
The term is also used for a chronically infected tract such as a passage between an abscess and the skin. It is however distinct from a fistula, which is a tract connecting two epithelialised surfaces.
Other sinuses in the body