1. An air-filled cavity in a dense portion of a skull bone: The sinuses are formed in four right and 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 also the mucous cannot drain out.

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. Etymology: "abscess, sore", from Middle Latin sinus, from Latin sinus, "bend, fold, curve".