trigger

(TRIH-ger) In medicine, a specific event that starts a process or that causes a particular outcome. For example, chemotherapy, painful treatments, or the smells, sounds, and sights that go with them may trigger anxiety and fear in a patient who has cancer. In allergies, exposure to mold, pollen or dust may trigger sneezing, watery eyes, and coughing.

An event that causes a "handler" to be run.