Excess fluid between the two membranes that envelop the lungs. These membranes are called the visceral and parietal pleurae. The visceral pleura wraps around the lung while 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 over the visceral and parietal pleurae and acts as a lubricant between the two membranes. Any significant increase in the quantity of pleural fluid is a pleural effusion.
The most common symptoms are chest pain and difficulty breathing (dyspnea). Many pleural effusions cause no symptoms but are discovered during the physical examination or seen on a chest X-ray, which is the most convenient way to confirm the diagnosis.

It is water gathered in the membrane of lungs