Noun. 1. forearm-the part of the superior limb between the elbow and the wrist. anconeous muscle, musculus anconeus-the muscle that extends the forearm and abducts the ulna in pronation of the wrist. accessory cephalic vein, vena cephalica accessoria-a vein that passes along the radial edge of the forearm and joins the cephalic vein near the elbow.