Music therapy is the planned use of music to meet therapeutic goals. The Australian Music Therapy Association has a rather good definition. The following comes from the AMTA website.
Music therapy is a research-based practice and profession in which music is used to actively support people as they strive to improve their health, functioning and wellbeing.
Music therapists incorporate a range of music making methods within and through a therapeutic relationship. They are employed in a variety of sectors including health, community, aged care, disability, early childhood, and private practice. Music therapy is different from music education and entertainment as it focuses on health, functioning and wellbeing.
Music therapists are committed to supporting people of any age and ability regardless of musical skill, culture or background.