Pleural effusion is the abnormal accumulation of fluid in the pleural space, which is the cavity between the two membranes (pleura) that surround the lungs. This condition can restrict respiratory functions by putting pressure on the lungs and lead to various symptoms. Commonly referred to as 'fluid in the lung,' this condition typically describes pleural effusion. It should not be confused with pleurisy, which means the inflammation of the pleural membranes; pleurisy is an inflammatory condition that can cause effusion. Pulmonary edema, on the other hand, is the accumulation of fluid within the air sacs (alveoli) inside the lung itself and represents a different medical condition from pleural effusion.