a part of a gun that causes the gun to fire when pressed

It's not clear who actually pulled the trigger. 不清楚究竟是谁扣动的扳机。

an event or situation, etc. that causes something to start

There are fears that the incident may be a trigger for more violence in the capital. 有人担心，这次事件可能在首都引发进一步的暴力活动。

something that causes someone to feel upset and frightened because they are made to remember something bad that has happened in the past

A trigger is something that sets off a flashback, transporting the person back to the traumatic event. 诱因引发往事的重现，将人带回到创伤性的事件中。

to cause something to start

Some people find that certain foods trigger their headaches. 有些人发现某些食物导致他们头疼。

Ultraviolet-B radiation triggers the skin to produce vitamin D. 紫外线B的辐射会促使皮肤产生维生素D。

The racial killings at the weekend have triggered off a wave of protests. 周末发生的种族屠杀引发了抗议浪潮。