Answer:
Definition: "A condition of persistent mental or emotional stress occurring as a result of injury or severe psychological shock"
Causes: Its often caused by severe injuries or experiences. For example, a majority of surviving soldiers from war tend to have PTSD. Other causes can include: abuse, traumatic experiences, assault etc.
Treatment: According the the NHS, treatments include: confronting your feelings and/or getting professional help. Many people have to go to psychologists and therapists