1) An accumulation of air or gas in the pleural space, which may occur spontaneously or as a result of trauma or a pathological process, or be introduced deliberately ( = pneumothorax, artificial). (Dorland, 27th ed)

2) A condition in which air or other gas is present in the pleural cavity and which occurs spontaneously as a result of disease or injury of lung tissue, rupture of air-filled pulmonary cysts, or puncture of the chest wall or is induced as a therapeutic measure to collapse the lung.