Acute stress disorder (ASD) refers to a new category of mental disorder in DSM-IV that is defined as a reaction occurring within 4 weeks following a traumatic experience and is characterized by dissociative symptoms, reexperiencing, avoidance, and marked anxiety or arousal.

Contrasts with posttraumatic stress disorder, which either lasts longer or has a delayed onset.

Other definition:
Acute Stress Disorder refers to an Anxiety disorder that develops after a traumatic event with symptoms such as depersonalization, numbing, dissociative amnesia, intense anxiety, hypervigilance, and impairment of everyday functioning. People with this disorder may re-experience the event and desperately avoid reminders of the trauma. These symptoms arise within the month following the trauma and last from days to weeks. Acute Stress Disorder is a relatively new diagnostic category and was added to the fourth (4th) Edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) in 1994 to distinguish time-limited reactions to trauma from the farther-reaching and longer-lasting post-traumatic stress disorder (PTSD).

Also, it is a severe reaction immediately following a terrifying event, often including amnesia about the event, emotional numbing, and derealization. Many victims later develop Posttraumatic Stress Disorder.

Acute Stress Disorder is also known as ASD.

Other definition:
Acute stress disorder refers to a psychological disturbance lasting up to one (1) month following stresses that would produce anxiety in anyone who experienced them.

Other definition:

Acute Stress Disorder refers to the anxiety and behavioral disturbances that develop within a month of exposure to extreme trauma. The symptoms of an Acute Stress Disorder usually begin during or shortly following the trauma. Examples of extreme traumatic events include rape or other severe physical assault, near-death experiences in accidents, witnessing a murder, and combat. Acute Stress Disorder also features a symptom of dissociation, which reflects a perceived detachment of the mind from the emotional state or even the body, is a critical feature. It is characterized by a sense of the world as a dreamlike or unreal place and may be accompanied by poor memory of the specific events. When severe, it is known as Dissociative Amnesia. Other features of Acute Stress Disorder include symptoms of generalized anxiety and hyperarousal, avoidance of situations or stimuli that elicit memories of the trauma, and persistent, intrusive recollections of the event via flashbacks, dreams, or recurrent thoughts or visual images. The diagnosis is changed to Posttraumatic Stress Disorder (PTSD) when the symptoms and behavioral disturbances of the Acute Stress Disorder persist for more than a month, and if these features are associated with functional impairment or significant distress to the sufferer.