Sinus

A sinus is a hollow air pocket behind the face. There are different pairs of sinuses, including the frontal sinuses behind the forehead, the ethmoid sinuses behind the eyes, the sphenoid sinuses behind the ethmoid sinuses, and the maxillary sinuses behind the cheeks.

The sinuses are part of the same system as the nose. When a sinus or the nose becomes inflamed, it produces the condition called rhinosinusitis.