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.

See also

• Sinus is a Latin name for the trigonometric function sine, a concept of mathematics. (See trigonometry.)