Post-traumatic stress disorder (PTSD) is a set of reactions that can develop in people who have experienced or witnessed a traumatic event that threatens their life or safety (or others around them). This could be a car or other serious accident, physical or sexual assault, war-related events or torture, or a natural disaster such as bushfire or flood.
Other life-changing events such as being retrenched, getting divorced, or the death of a family member are very distressing, and may cause mental health problems, but are not considered traumatic events that can cause PTSD.