A psychological disorder triggered by extreme trauma that the mind of the sufferer is unable to cope with, causing them to retreat into a much reduced state of functioning.
Shell shock marked the first real recognized psychological disorder and was first noted during The Great War as many soldiers were coming to the war hospitals with nothing physically wrong with them but still completely unfit for war. It was at this time that the doctors in these institutions began to look to Freud's theories that certain mental traumas could produce effects just as severe as physical ones.
Shell shock victims, especially those of war, often exhibit a twitch in a part of their body that is associated with the trauma. For soldiers this means that if they saw someone's leg blown off then they would develop a tic in their leg. If it was the face, they would have a facial tic, and so on and so forth.