Sinus, SY nuhs, is a cavity in the bodies of animals, including human beings. When used alone, the term sinus generally refers to a hollow, air-filled space in the front of the skull. The human skull contains four groups of sinuses. Doctors sometimes call these groups the paranasal sinuses, because they all connect with the nasal cavity. Each group of sinuses takes its name from the bones of the skull in which it is found.