The symptomatic return of repressed childhood sexual trauma. The two main forms of hysteria are 1) conversion hysteria, in which the symptoms are manifested on the body (eg. psychosomatic illness); and 2) anxiety hysteria, in which one feels excessive anxiety because of an external object (eg. phobias).
Hysteria is a diagnostic label applied to a state of mind, one of unmanageable fear or emotional excesses. The fear is often centered on a body part, most often on an imagined problem with that body part (disease is a common complaint). People who are "hysterical" often lose self-control due to the overwhelming fear. 
The term originates with the Greek medical term, hysterikos. This referred to a supposed medical condition, peculiar to women, caused by disturbances of the uterus, hystera in Greek. The term hysteria was coined by Hippocrates, who thought that the cause of hysteria was irregular movement of blood from the uterus to the brain.
The same general definition, or under the name female hysteria, came into widespread use in the middle and late 19th century to describe what is today generally considered to be sexual dissatisfaction. Typical "treatment" was massage of the patient's genitalia by the physician and later vibrators or water sprays to cause orgasm. By the early 1900s the practice, and usage of the term, had fallen from use, until it was again popularised when the writings of Sigmund Freud became known and influential in Britain and the USA in the 1920s. The Freudian psychoanalytic school of psychology uses its own, somewhat controversial, ways to treat hysteria.
The knowledge of hysterical processes was advanced by the work of Jean-Martin Charcot, a French neurologist. However, many now consider hysteria to be a 'legacy diagnosis' (i.e.: a catch-all junk diagnosis), particularly due to its long list of possible manifestations: one Victorian physician catalogued 75 pages of possible symptoms of hysteria and called the list incomplete.
A form of neurosis for which no physical diagnosis can be found and in which the symptoms presented are expressive of an unconscious conflict. In conversion hysteria, the symptoms usually take a somatic form (hysterical paralysis, irritation of the throat, coughs); in anxiety hysteria or phobia, the cause of the anxiety is a particular external object. Hysteria has been explained in many different ways over the centuries; the most influential aetiology or causal explanation to have been put forward in the twentieth century is that supplied by Freud's psychoanalysis.
Freud's initial thesis (1896) was the so-called seduction theory which held that hysteria and other neuroses originate in an experience of actual sexual abuse that has been repressed, but which can be recovered. Freud initially used hypnosis to recover these memories, but abandoned it in favor of what Anna O. so felicitously termed the talking cure. Freud subsequently revised his original seduction theory in the course of a long correspondence with his collaborator Fliess and argued that hysteria stemmed from unconscious fantasies about incest that were bound up with oedipus complex, though he never denied either the existence or the pathogenic effects of real abuse.