Sinusitis is when those hollow spaces around your nose swell and hurt. The lining inside the sinus gets inflamed, so the space cannot drain, and you feel pressure in the face, often with a blocked nose. Doctors may call it a sinus infection; that is the name most families use. It can follow a cold and fade in a few days, or hang around for a long time. It is a clinic word, a little technical, but parents use it freely once a doctor has said it. It is not the same as a simple runny nose — though actually the two often travel together, which is why people mix them up.