Sinusitis is an of the paranasal sinuses. The full medical term for sinusitis is "rhinosinusitis" ("rhino-" meaning "nose"), because it affects the mucous membranes lining both the nose and the sinuses.
The paranasal sinuses are part of the upper airways, and are connected to the nasal cavity. They are made up of several cavities in the skull found from the forehead down to the teeth of the upper jaw. Depending on where they are, these cavities are known as the frontal sinuses, the sphenoid sinus, the ethmoid cells and the maxillary sinuses. The paranasal sinuses are lined with mucous membranes that have tiny hairs on them (ciliated epithelium). These mucous membranes produce a secretion that runs down through the nose and throat.