What is the term for the tendon that is a broad, flat sheet?

The term for a tendon that is a broad, flat sheet is indeed aponeurosis. An aponeurosis serves a critical role in the anatomy of muscles by providing an extensive surface area for muscle attachment to the skeleton or other muscles. This broad structure allows for a more distributed force across the area where muscles connect, facilitating a more effective pull during muscle contraction.

In contrast to an aponeurosis, endomysium refers to the connective tissue that surrounds individual muscle fibers, providing structural integrity and facilitating nutrient exchange. Perimysium is the connective tissue that surrounds a bundle of muscle fibers, or fascicle, thus serving a slightly different function in supporting and organizing muscle tissues. Fascia is a more general term that refers to the connective tissue that encases muscles and other structures, but it does not specifically describe the flat sheet-like structure characteristic of an aponeurosis.

Overall, aponeuroses are crucial in muscle function and organization, distinguishing them as a specific type of tendon ideal for certain anatomical and functional roles.