A sinus is a pouch or cavity in any organ or tissue, or an abnormal cavity or passage caused by the destruction of tissue. The term is applied to a large number and variety of structures in the human body, such as:-
- The air cavities in the cranial bones, especially those near the nose and connecting to it. These are the paranasal sinuses and are usually what is meant when a person's "sinuses" are referred to.
- A dilated portion of a blood vein, such as inside the braincase.
- A chronically infected tract such as a fistula.