Problem: It is claimed that there are more than 400 kinds of psychotherapy. Is this true? What are the common characteristics that make these the same kind of practice? How does one differentiate between what is psychotherapy and what is not (e.g. psychiatry, coaching, education, philosophical counseling, meditation)? A clear definition would help us answer these questions. Aim: This is an attempt to define the concept of “psychotherapy”. I start by critically scrutinizing several definitions suggested by other writers and then present and argue for my own suggestion. Method: The method used is philosophical, or conceptual. This means that I try to state the necessary and jointly sufficient characteristics of the term in question. The starting point is that all kinds of psychotherapy share some characteristics, without which they would not qualify as psychotherapy, and that it is these core characteristics that constitute the practice. Result: Psychotherapy is the interaction between two or more persons, a client and a therapist, where the latter is trained (at least to some degree), based on some kind of general theory of therapy and personality change, where the aim is to create significant and lasting positive quality-of-life-related mental changes, involving several aspects of the person’s mental “faculties” (cognition, emotion, perception, volition, attitudes, or values), where the means to reach these goals are mainly verbal (but sometimes also non-verbal), and this interacting is primarily what leads to the positive change. A practice that does not fulfill these “minimal” requirements will not count as psychotherapy.