Definition "trigger" in Collins English Dictionary & Thesaurus, spelling "trigger"

trigger (triggers plural & 3rd person present) (triggering present participle) (triggered past tense & past participle )

1 n-count The trigger of a gun is a small lever which you pull to fire it.
A man pointed a gun at them and pulled the trigger.

2 n-count The trigger of a bomb is the device which causes it to explode.
oft N n
...trigger devices for nuclear weapons.

3 verb To trigger a bomb or system means to cause it to work. (=activate)
The thieves must have deliberately triggered the alarm and hidden inside the house... V n

4  verb If something triggers an event or situation, it causes it to begin to happen or exist.
...the incident which triggered the outbreak of the First World War...  V n
  Trigger off means the same as trigger., phrasal verb
It is still not clear what events triggered off the demonstrations.  V P n (not pron), Also V n P

5 n-count If something acts as a trigger for another thing such as an illness, event, or situation, the first thing causes the second thing to begin to happen or exist.
oft N for n
Stress may act as a trigger for these illnesses.