There are a lot of attempts to define psychotherapy. But a short version would be of advantage for the field. In our paper "Der Psychotherapeut als Scientist-Practitioner - Vom Bachelor und Master über das 1. und 2. Staatsexamen zur Approbation und sozialrechtlichen Zulassung) we tried to define it either as "psychological treatment of any disorder / disease" or alternatively as "treatment of mental disorders". Both ways contain advantages and problems. The first one for example has the problem of overlapping with some fields of medicine, especially psychosomatic medicine. And the second would include the whole discipline of psychiatry.
What do you think is best? Is it possible to combine our two definitions? Do you have alternative or better (short) definitions?