Final answer:
A flat sheet of connective tissue that attaches muscle to bone is called an aponeurosis.
Step-by-step explanation:
A flat sheet of connective tissue that extends beyond the muscle fibers to attach the muscle to bone is known as an aponeurosis. This connective tissue is similar to a tendon but it differs in being a broader, flatter structure. In skeletal muscles, when the collagen of the three tissue layers, or the mysia, does not interweave with a tendon, it may instead fuse with an aponeurosis or to the fascia. An example of an aponeurosis can be seen in the lower back where the latissimus dorsi muscles, also known as "lats", fuse into this flat, wide connective tissue to aid in movement of the skeleton.