Pleural effusion: Excess fluid between the two membranes that cover the lungs (the visceral and parietal pleurae) separating the lungs from the chest wall. A small quantity of fluid is normally 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 of pleural effusion are chest pain and painful breathing (pleurisy). Many pleural effusions cause no symptoms but are discovered during physical examination or detected on chest X-rays; X-ray is the most convenient way to confirm the diagnosis. Pleural effusion can be caused by heart and kidney failure, hypoalbuminemia (low levels of albumin in the blood), infections, pulmonary embolism, and malignancies.