Triggering is when something that is read, seen, heard, etc. causes someone who has suffered a traumatic event to once again feel the emotions they felt during the time of their traumatic experience. It can sometimes make someone feel helpless or afraid and can be intense that the person who experienced the trauma can't stop reading, watching, listening.

Triggering has several meanings in English. It is a term used to describe an event caused by another event previously. For example, the earthquake triggered (or caused) the landslide. You can say it the other way around as, The landslide was triggered by the earthquake. When you talk about people, you might say something like 'ill health triggered a heart attack', or 'pollen can trigger hey fever in some people'. A trigger is also the part of a gun that fires the bullet, so the term 'triggering' is often heard in military terms to mean who you are aiming at.