Art Therapy can be defined many different ways mainly depending on who you ask. I typically tell people that it is much easier to show people what Art Therapy is rather than explain it because the essence of what Art Therapy is, is truly beyond words. But obviously we need to be able to communicate what it is that we do as well. Below I have collected some great definitions of Art Therapy…
Art therapy is a form of expressive therapy that uses art materials, such as paints, chalks, and markers. Art therapy combines traditional psychotherapeutic theories and techniques with an understanding of the psychological aspects of the creative process, especially the affective properties of the different art materials.
From the Art Therapy Without Borders website: Art has the potential to change lives and in profound ways. When words are not enough, we turn to images and symbols to tell our stories. And in telling our stories through art, we can find a path to health and wellness, emotional reparation, recovery, and ultimately, transformation.
From the Art Therapy Alliance: Art therapy is the deliberate use of art-making to address psychological and emotional needs. Art therapy uses art media and the creative process to help in areas such as, but not limited to: fostering self-expression, enhancing coping skills, managing stress, and strengthening a sense of self.
From the book Expressive Therapies : The purpose of art therapy is much the same as in any other psychotherapeutic modality: to improve or maintain mental health and emotional well-being. But whereas some of the other expressive therapies utilize the performing arts for expressive purposes, art therapy generally utilizes drawing, painting, sculpture, photography, and other forms of visual art expression. For that reason art therapists are trained to recognize the nonverbal symbols and metaphors that are communicated within the creative process, symbols and metaphors which might be difficult to express in words or in other modalities. Art making is seen as an opportunity to express oneself imaginatively, authentically, and spontaneously, an experience that, over time, can lead to personal fulfillment, emotional reparation, and transformation. This view also holds that the creative process, in and of itself, can be a health-enhancing and growth producing experience.
From Art Therapy Sourcebook  and Art Therapy Without Borders: Although art therapists have generated many specific definitions of art therapy, most of them fall into one of two general categories. The first involves a belief in the inherent healing power of the creative process of art making. This view embraces the idea that the process of making art is therapeutic; this process is sometimes referred to as art as therapy. Art making is seen as an opportunity to express oneself imaginatively, authentically, and spontaneously, an experience that, over time, can lead to personal fulfillment, emotional reparation, and transformation.
The second definition of art therapy is based on the idea that art is a means of symbolic communication. This approach, often referred to as art psychotherapy, emphasizes the products–drawings, paintings, and other art expressions–as helpful in communicating issues, emotions, and conflicts. The art image becomes significant in enhancing verbal exchange between the person and the therapist and in achieving insight; resolving conflicts; solving problems; and formulating new perceptions that in turn lead to positive changes, growth, and healing. In reality, art as therapy and art psychotherapy are used together in varying degrees. In other words, both the idea that art making can be a healing process and that art products communicate information relevant to therapy are important.
These definitions were taken form the Art Therapy Without Borders website: http://www.atwb.org/what-is-art-therapy/