Effusion, pleural

Effusion, pleural: Outpouring of fluid between the two layers of the pleural membranes that cover the lungs. The term "effusion" comes from the Latin "effusio" meaning a pouring out. A pleural effusion is thus, literally, a pouring out of fluid into the pleural space. By comparison, a pericardial effusion is an outpouring of fluid within the fibrous sac (the pericardium) that surrounds the heart.