The sphenoidal sinus is a small, air-filled cavity located in the skull, behind the eyes and nasal cavity. Synonyms for this anatomical feature include the sphenoid sinus, sinus sphenoidalis or sinus of the sphenoïd bone. It is part of the paranasal sinuses system and works together with the other sinuses to regulate air pressure during breathing. Other synonyms for this structure include air cells of the sphenoid bone and sphenoid air sinus. Despite the differences in the terms used to reference it, the sphenoidal sinus serves an important function in the human body, helping to filter, warm, and humidify the air we breathe.