Sinus refers to hollow spaces inside your skull, located around your nose, cheeks, and forehead. These air-filled cavities connect to your nasal passages through small openings and are lined with a thin layer of mucus that helps trap dust, germs, and other particles before they reach your lungs.

You have several pairs of sinuses with scientific names like the maxillary sinuses (in your cheekbones) and frontal sinuses (above your eyebrows). When you're healthy, you never notice them. But when you catch a cold or have allergies, the lining of your sinuses can swell and produce extra mucus, making your face feel heavy and achy. This uncomfortable condition is called sinusitis, also known as a sinus infection.

Scientists aren't entirely sure why we have sinuses, though they may help make our skulls lighter, warm the air we breathe, or improve our voices by creating resonating chambers. When a doctor asks if you have sinus pressure, they want to know if these spaces feel blocked or painful.