Posttraumatic stress disorder (PTSD) is an anxiety disorder that can develop after exposure to one or more traumatic events that threatened or caused grave physical harm. PTSD affects over 7.8 million people.

It is a severe and ongoing emotional reaction to an extreme psychological trauma. This stressor may involve someone's actual death, a threat to the patient's or someone else's life, serious physical injury, an unwanted sexual act, or a threat to physical or psychological integrity, overwhelming psychological defenses.

In some cases it can also be from profound psychological and emotional trauma, apart from any actual physical harm. Often, however, incidents involving both things are found to be the cause.

PTSD is a condition distinct from traumatic stress, which has less intensity and duration, and combat stress reaction, which is transitory. PTSD has also been recognized in the past as railway spine, stress syndrome, shell shock, battle fatigue, traumatic war neurosis, or post-traumatic stress syndrome (PTSS).

Diagnostic symptoms include reexperience such as flashbacks and nightmares, avoidance of stimuli associated with the trauma, increased arousal such as difficulty falling or staying asleep, anger and hypervigilance. Per definition, the symptoms last more than six months and cause significant impairment in social, occupational, or other important areas of functioning (e.g. problems with work and relationships.)