Trigger

From Wikinfo

A trigger is a basic process which either forms the preconditions for a subsequent consequent event or process or is the precipitating cause of the event. For example, some presently unknown set of conditions is believe 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 is an example of a trigger.

Manufactured devices such as guns may incorporate trigger mechanisms.

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