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A trigger is a part of the mechanism of a gun. Pulling the trigger sets off the chain of events that fires the gun.

The wider concept of a trigger refers either to the preconditions for a subsequent consequent event or process, or to the precipitating cause of the event. The term is an analogy to the trigger of a gun. For example, some presently unknown set of conditions is believed to be the trigger for the commencement of an ice age. Or in the case of a supersaturated solution addition of a seed crystal will trigger the precipitation of the dissolved material. The example given in chaos theory of a butterfly causing a great weather disturbance describes a trigger.

Systems such as databases may incorporate trigger mechanisms whereby certain coded procedures run whenever an event (such as an insert, an update or a delete) occurs.

Following this analogy, an event which causes an allergic reaction or other untoward psychological/physical event is also referred to as a trigger. For example, exposure to cat dander may trigger an asthma attack in an allergic person, and persons with post-traumatic stress disorder may experience extreme distress after being exposed to their personal trigger.