The word depth is used here to signify the type of psychotherapy that deals with the reality of the unconscious, the objective psyche. The term psychotherapy is a product of two root words: psyche originally meant soul or life spirit; the Greek verb therapeuein means to tend or render service, and the original usage of the work was to render service to the gods in their temples. So, in the temples of antiquity, therapeueine referred to the careful attendance to cult(ic) worship and religions ceremonies. Then, by extension, that verb came to refer to the care, attention, and treatment of patients in a medical setting. Our understanding of the modern equivalent, therapy, is deepened by reflecting on the root meaning. It implies that service to the soul—psyche—is not just a secular affair; it is more than an ego-dominated business, for it has a transpersonal dimension.
Edinger, E. (1997). The vocation of depth psychotherapy. Psychological perspectives: A journal of global consciousness, integrating psyche, soul, and nature, 35, p. 10.