What defines the epicardium of the heart?

The epicardium is indeed defined as a protective outer layer of the heart. It is the visceral layer of the serous pericardium and serves important functions, including providing lubrication to reduce friction between the heart and surrounding structures during the heartbeat. This outer layer is also involved in the production of pericardial fluid and helps protect the heart from physical damage.

In the context of the heart's anatomy, the term "epicardium" is often used to describe the layer that lies directly on the surface of the heart, distinguishing it from other layers such as the myocardium (the thick muscular layer responsible for contraction) and the endocardium (the inner lining that covers the valves and the chambers). Thus, the defining characteristic of the epicardium as a protective outer layer accurately encapsulates its role in cardiac physiology.