The term psyche is central to the field of mental health and dysfunction. And yet the term is seldom talked about directly, and is seldom elaborated by the professionals who function under its rubric. This article elaborates this term, and sets forth a definition for the purposes of psychology and psychodynamics as science in the sense of the establishment of psychological knowledge. Psyche is defined in this article as a hypothetical construct, as outlined in the classic article in the theory of psychological science by MacCorquodale and Meehl (1948). Psyche is to be distinguished from the concept of mind, which is identified here as a strictly phenomenological (subjective) concept. Use of psyche in this way allows it to be utilized in descriptions of its dynamics (psychodynamics), as a central concept in a science which utilizes phenomenological (subjective rather than objective) data.