What is the name of the membrane sac covering the outside of the heart?

The membrane sac that covers the outside of the heart is known as the pericardium. This double-layered structure serves several important functions, including protecting the heart from infection and providing a lubricated environment that reduces friction as the heart beats. The outer layer, called the fibrous pericardium, is tough and helps anchor the heart in place within the thoracic cavity, while the inner layer, called the serous pericardium, produces pericardial fluid that further aids in reducing friction.

Other options refer to different structures or membranes: the peritoneum is associated with the abdominal cavity, pleurisy relates to inflammation of the pleura (the membrane surrounding the lungs), and the endocardium lines the interior of the heart chambers. Therefore, the pericardium is the specific term used for the membrane directly surrounding the heart itself.