Pleura: One of the two membranes around the lungs. These two membranes are called the visceral and parietal pleurae. The visceral pleura envelops the lung, and the parietal pleura lines the inner chest wall. There is normally a small quantity (about 3 to 4 teaspoons) of fluid that is spread thinly between the visceral and parietal pleurae. The pleural fluid acts as a lubricant between the two membranes.