What does "trigger" mean?

Trigger: (Mech.) A piece, as a lever, which is connected with a catch or detent as a means of releasing it; especially (Firearms), the part of a lock which is moved by the finger to release the cock and discharge the piece. Trigger fish (Zoöl.), a large plectognath fish (Balistes Carolinensis or B. capriscus) common on the southern coast of the United States, and valued as a food fish in some localities. Its rough skin is used for scouring and polishing in the place of sandpaper. Called also leather jacket, and turbot.