Hysteria is an obsolete medical term that is still used colloquially to refer to a state of extreme fear or emotion and the resultant irrational behavior. The term was originally employed to describe women who acted irrationally due to a supposed disturbance of the uterus. In 1980, the American Psychiatric Association (APA) replaced the diagnosis "hysterical neurosis, conversion type" with "conversion disorder."

The word hysteria is derived from the Greek for "uterus." According to ancient medical thought, women who refrained from sexual intercourse for a prolonged period suffered from mental disturbance as a result of the uterus retreating into the body and compressing the other organs. This theory was revived in 19th century medicine and again in the 1920s through the influence of Freud's psychological theories. Induced orgasm was used as treatment for supposedly hysterical women. The notion of so-called female hysteria has since been discredited.

In modern psychological thought, there are two types of hysteria: somatoform and dissociative. Somatoform hysteria is characterized by physical, or psychosomatic, symptoms. One example is conversion disorder, in which neurological symptoms such as paralysis, pain, and fits are present with no neurological cause. Other examples include body dysmorphic disorder, chronic fatigue syndrome, and hypochondria. Patients with somatoform hysteria are not faking their illnesses; rather, the symptoms have a psychological, or inorganic, cause instead of a biological one.

Dissociative hysteria is characterized by a fugue state, otherwise known as amnesia, in which the patient experiences a loss of identity to some degree. Memory loss and changes in personality are common symptoms. Like somatoform hysteria, a fugue state is not intentional evasion, but rather an actual loss of recall due to psychological stress or trauma.

Current psychiatric theory holds that hysteria is the body's attempt to deal with psychological distress. There may be a benefit to the patient, such as avoidance of whatever is causing the distress; i.e. one becomes too sick to continue working at a stressful job or living on one's own. Sometimes, however there is no apparent benefit; for example, a person experiencing symptoms of poisoning out of the mistaken belief that he or she has been poisoned.