Pericardium: The conical sac of fibrous tissue that surrounds the heart and the roots of the great blood vessels. The pericardium's outer coat (the parietal pericardium) is tough and thickened, loosely cloaks the heart, and is attached to the central part of the diaphragm and the back of the breastbone. Its inner coat (the visceral pericardium or epicardium) is double, with one layer closely adherent to the heart and the other lining the inner surface of the outer coat. The intervening space between these layers is filled with pericardial fluid. This small amount of fluid acts as a lubricant to allow normal heart movement within the chest.