Sinus:1. A sinus is an air-filled space within the solid structure of a skull bone, reducing its overall weight. These cavities are arranged in four pairs, symmetrically on the right and left sides. Located behind the forehead are the frontal sinuses, while the maxillary sinuses are found behind the cheeks. The sphenoid and ethmoid sinuses are situated deeper within the skull, behind the eyes and maxillary sinuses. These cavities are lined with cells that secrete mucus. Air reaches the sinuses through small openings in the bone known as ostia. If an ostium becomes blocked, air cannot enter, and mucus cannot exit. See also: related topics.2. A sinus can also refer to a channel that allows the movement of blood or lymph fluid without being a blood or lymphatic vessel, such as the sinuses found in the placenta.3. It may also describe a passage or tract leading to a cavity that can be filled with fluid or other substances.The term "sinus" originates from the Latin word "sinus," meaning "curve, fold, or hollow," which also led to the word "sinuous."