In psychology a complex is an important group of unconscious associations connected by a recognizable theme, or a strong unconscious impulse lying behind an individual's otherwise mysterious condition: the detail varies widely from theory to theory. However their existence is widely agreed upon in the area of depth psychology at least, being instrumental in the systems of both Freud and Jung. They are generally a way of mapping the psyche, and are crucial theoretical items of common reference to be found in therapy.
The term "complex," or "feeling-toned complex of ideas," was adopted by Carl Jung when he was still a close associate of Sigmund Freud. (Theodor Ziehen is credited with coining the term in 1898.) Jung described a "complex" as a 'node' in the unconscious; it may be imagined as a knot of unconscious feelings and beliefs, detectable indirectly, through behavior that is puzzling or hard to account for.
Jung found evidence for complexes very early in his career, in the word association tests conducted at the Burghölzli, the psychiatric clinic of Zurich University, where Jung worked from 1900-1908. In the word association tests, a researcher read a list of words to each subject, who was asked to say, as quickly as possible, the first thing that came to mind in response to each word. Researchers timed subjects' responses, and noted any unusual reactions--hesitations, slips of the tongue, signs of emotion. Jung was interested in patterns he detected in subjects' responses, hinting at unconscious feelings and beliefs.
In Jung's theory, complexes may be conscious, partly conscious, or unconscious. They may be related to traumatic experiences, or not. There are many kinds of complex, but at the core of any complex is a universal pattern of experience, or archetype. Some of the key complexes Jung wrote about were the anima (a node of unconscious beliefs and feelings in a man's psyche relating to the opposite gender) and animus (the corresponding complex in a woman's psyche); and the shadow (Jung's term embracing any aspect of psyche which has been excluded from conscious awareness). Many Jungian complexes appear in complementary pairs: for example, the puer, or eternal youth, often appears in relationship to the senex, or archetypal old man. A puer complex might manifest as an individual's unconscious dread of growing up, of losing one's romantic ideals or freedom; a senex complex, by contrast, might be seen in a person who, without seeming to understand why, is driven to act out an "old man" role, in creative or destructive ways. Only when a complex results in destructive behavior would it be seen as pathological; otherwise, a Jungian view of psyche accepts the presence of diverse complexes in ordinary health.
Jung defined the ego as one complex among many. "By ego I understand a complex of ideas . . . Hence I also speak of an ego-complex. . . . the ego . . . merely being one complex among many."
Jung understood complexes as splinter psyches. "Today we can take it as moderately certain that complexes are in fact 'splinter psyches'."
One of the key differences between Jungian and Freudian theory is that Jung's thought posits several different kinds of complex, and emphasizes duality or plurality rather than unity as a basic condition of the human psyche. Freud held that the Oedipus complex was universal--reflecting developmental challenges that face every child--and was the central complex in most or all psychopathology.
Once Jung broke from Freud and the two men went their own ways, forming their own disciplines behind them, there was briefly a movement in some of Freud's circle to remove all of Jung's work and terminology from their school of psychoanalysis. Freud himself however refused, and so the name "complex" stayed.