A trigger in psychology is a stimulus such as a smell, sound, or sight that triggers feelings of trauma. People typically use this term when describing posttraumatic stress (PTSD).

WHAT IS A TRIGGER?
A trigger is a reminder of a past trauma. This reminder can cause a person to feel overwhelming sadness, anxiety, or panic. It may also cause someone to have flashbacks. A flashback is a vivid, often negative memory that may appear without warning. It can cause someone to lose track of their surroundings and “relive” a traumatic event.

Triggers can take many forms. They may be a physical location or the anniversary of the traumatic event. A person could also be triggered by internal processes such as stress.