Post-traumatic stress disorder (PTSD) is an emotional illness that develops as a result of a terrifying, life-threatening, or otherwise highly unsafe experience. Sufferers re-experience the traumatic event or events in some way, tend to avoid places, people, or other things that remind them of the event (avoidance), and are exquisitely sensitive to normal life experiences (hyper arousal).
See: http://www.medicinenet.com/posttraumatic_stress_disorder/article.htm
