Sinus: 1. An air-filled cavity in a dense portion of a skull bone. The sinuses decrease the weight of the skull. The sinuses are formed in four right-left pairs. The frontal sinuses are positioned behind the forehead, while the maxillary sinuses are behind the cheeks. The sphenoid and ethmoid sinuses are deeper in the skull behind the eyes and maxillary sinuses. The sinuses are lined by mucous-secreting cells. Air enters the sinuses through small opening in bone called ostia. If an ostium is blocked, air cannot pass into the sinus and likewise mucous cannot drain out. See also: Sinusitis.
2. A channel permitting the passage of blood or lymph fluid that is not a blood or lymphatic vessel, such as the sinuses of the placenta.
3. A tract or fistula leading to a cavity which may be filled with pus.
The word was borrowed from the Latin noun "sinus," which means "curve, fold, or hollow." The same root gave rise to "sinuous".