A sinus is a hollow space within a bone or tissue, commonly found in the human skull. These air-filled cavities are located around the nasal passages and help to lighten the weight of the skull, improve voice resonance, and produce mucus that keeps the nasal passages moist. The most well-known sinuses are the maxillary, frontal, ethmoid, and sphenoid sinuses. Sinus infections, or sinusitis, occur when these cavities become inflamed or infected, leading to symptoms like congestion, facial pain, and headaches. Proper sinus function is essential for respiratory health.