Trauma is an event where your survival smoke alarm—the amygdalae—went off and by doing so signaled your involuntary fight/flight/freeze response to go into immediate effect. Whether or not the traumatic event was truly life-threatening is immaterial. The fact that your being—in an effort to keep you alive—deemed the experience worthy of setting off the alarm bells is what counts.
Trauma can be caused by someone physically and/or emotionally abusing you. It can be you witnessing a horrible event. It can also be losing a loved one or having a serious illness. The same experience could be traumatic for one person and not traumatic for another. Also, trauma can be a seen as a mild and harmless incident for an outsider, but a traumatizing event for the person experiencing it. Trauma is a thoroughly subjective experience.
Some people are able to release the stress of their traumatic incident. They are able to resolve it shortly after the event takes place and suffer few effects from the experience. Many, however, absorb and hold onto the unresolved trauma in their bodies, minds, emotions and spirits. Also, if someone has experienced chronic trauma as a child or adult, he or she is much more vulnerable and less resilient when suffering a new trauma compared to someone who has no history of trauma.