Post traumatic stress isorder (PTSD) is an anxiety disorder that can develop after exposure to one or more terrifying events in which grave physical harm occurred or was threatened. It is a severe and ongoing emotional reaction to an extreme psychological trauma. This stressor may involve someone's actual death or a threat to the patient's or someone else's life, serious physical injury, or threat to physical and/or psychological integrity, to a degree that usual psychological defenses are incapable of coping. In some cases it can also be from profound psychological and emotional trauma, apart from any actual physical harm. Often, however, the two are combined.