Sinusitis is an inflammation of the nasal sinuses.
A sinus is a cavity (air chamber) within the bone. These are four sets of open spaces within the bones of the skull. They are above the eyes (frontal sinuses), to either side of the nose inside the cheekbones (maxillary sinuses), behind the bridge of the nose (sphenoid sinuses) and in the upper nose (ethmoid sinuses).
Read the entire article Sinus Infections or Sinusitis – Inflammation of the Nasal Sinuses