Post-traumatic Stress Disorder (PTSD) is an anxiety disorder that can develop after exposure to a terrifying event or ordeal in which grave physical harm occurred or was threatened.

PTSD 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.

With PTSD it is important to make a distinction between PTSD and Traumatic stress, which is a similar condition, but of less intensity and duration.

The condition has also been known historically or colloquially as shell shock, traumatic war neurosis, or post-traumatic stress syndrome (PTSS).