The pericardium is the outer layer of tissue that forms the wall of what human organ?

The pericardium (from the Greek περί, "around" and κάρδιον, "heart") is a tough double layered fibroserous sac which covers the heart. The space between the two layers of serous pericardium, the pericardial cavity, is filled with serous fluid which protects the heart from any kind of external jerk or shock. There are two layers to the pericardial sac: the outermost fibrous pericardium and the inner serous pericardium.

The pericardium fixes the heart to the mediastinum, gives protection against infection, and provides the lubrication for the heart.