Post-traumatic stress disorder (PTSD) is a term for certain severe psychological consequences of exposure to stressful events that the person experiences as highly traumatic. Clinically, such events involve actual or threatened death, serious physical injury, or a threat to physical and/or psychological integrity, to a degree that usual psychological defenses are incapable of coping with the impact. It is occasionally called post-traumatic stress reaction to emphasize that it is a result of traumatic experience rather than a manifestation of a pre-existing psychological weakness. The presence of a PTSD response is influenced by the intensity of the experience, its duration, and the individual person involved.

PTSD is thought to be primarily an anxiety disorder (possibly closely related to panic disorder) and should not be confused with normal grief and adjustment after traumatic events.

PTSD may be triggered by an external factor or factors. Its symptoms can include the following: nightmares, flashbacks, emotional detachment or numbing of feelings (emotional self-mortification or dissociation), insomnia, avoidance of reminders and extreme distress when exposed to the reminders ("triggers"), loss of appetite, irritability, hypervigilance, memory loss (may appear as difficulty paying attention), excessive startle response, clinical depression, and anxiety.