Sinusitis: Inflammation of the lining membrane in any of the hollow areas (sinuses) of the skull around the nose. Sinusitis may be caused by anything that interferes with air flow into the sinuses and the drainage of mucous out of the sinuses. The sinus openings, called ostia, may be obstructed by swelling of the tissue lining the ostia and adjacent nasal passage tissue; for example, from colds, allergies, and tissue irritants (nasal sprays, cocaine, cigarette smoke). Less commonly, sinuses can become obstructed by tumors or growths. Stagnated mucous then provides a perfect environment for bacterial infection. The common symptoms of sinusitis include headache; facial tenderness or pain; fever; cloudy, discolored nasal drainage; a feeling of nasal stuffiness; sore throat; and cough. Acute sinusitis is usually treated with antibiotic therapy. Chronic forms of sinusitis require long courses of antibiotics and may require a sinus drainage procedure.