According to the American Dance Therapy Association (ADTA): Based on the understanding that the body and mind are interrelated, dance/movement therapy (D/MT) is defined as the psychotherapeutic use of movement to further the emotional, cognitive, physical, and social integration of the individual. Dance/movement therapy is practiced in mental health, rehabilitation, medical, educational, and forensic settings, and in nursing homes, day care centers, disease prevention, and health promotion programs. The dance/movement therapist focuses on movement behavior as it emerges in the therapeutic relationship. Expressive, communicative, and adaptive behaviors are all considered for both group and individual treatment. Body movement as the core component of dance simultaneously provides the means of assessment and the mode of intervention for dance/movement therapy.

I often define D/MT to clients as psychotherapy that is not limited to talking but encompasses the full range of human expression, including movement such as gestures and or postures, drawing, writing, drama, music and other expressions that can have a therapeutic benefit for the client(s).

See also:
Wikipedia Dance Therapy
ADTA Fact Sheet