A sinus is a hollow cavity or space within a bone or tissue, commonly found in the human skull. These air-filled spaces 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 major sinuses include the frontal sinus, maxillary sinus, ethmoid sinus, and sphenoid sinus. Sinus health is important, as infections or blockages can lead to conditions like sinusitis, which causes inflammation and discomfort. Symptoms of sinus issues may include nasal congestion, facial pain, and headaches. Maintaining good nasal hygiene and seeking medical advice when necessary can help prevent sinus-related problems.