Trigger

n.
  1. A catch to hold the wheel of a carriage on a declivity.
  2. (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.
    3. Trigger fish (Zoöl.),
    a large plectognath fish (Balistes Carolinensis or Balistes 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.