There is no ‘accepted’ definition, according to HSE Research Report 323 Obstacles to recovery from msds in industry (2005), but the following description may be helpful:
Musculo-skeletal disorders (MSDs) are injuries and disorders of the muscles, nerves, tendons, ligaments, joints, cartilage and spinal disc; examples include carpal tunnel syndrome, rotator cuff tendonitis, and tension neck syndrome. (see ergoweb)
See also HSE's MSD home page which states:
Musculo-skeletal disorders (MSDs) are the most common occupational illness in Great Britain, affecting some 1.0 million people a year. They include problems such as low back pain, joint injuries and repetitive strain injuries of various sorts.
MSDs can affect many parts of the body and can be caused by a wide variety of occupational tasks. They also account for a high proportion of work-related absence, making them worthy of extra attention in order to reduce injuries and help maintain a productive workforce.