Source : Webster's Revised Unabridged Dictionary (1913)

Trigger \Trig"ger\, n. [For older tricker, from D. trekker, fr.
   trekken to draw, pull. See {Trick}, 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.

   {Trigger fish} (Zo["o]l.), 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}.

Source : WordNet®

trigger
     v 1: put in motion or move to act; "trigger a reaction"; "actuate
          the circuits" [syn: {trip}, {actuate}, {activate}, {set
          off}, {spark off}, {spark}, {trigger off}, {touch off}]
     2: release or pull the trigger on; "Trigger a gun"

trigger
     n 1: lever that activates the firing mechanism of a gun [syn: {gun
          trigger}]
     2: a device that activates or releases or causes something to
        happen
     3: an act that sets in motion some course of events [syn: {induction},
         {initiation}]

Source : Free On-Line Dictionary of Computing

trigger
     
         An action causing the automatic invocation of a
        procedure, for instance to preserve {referential integrity}.
        A triggers goes into effect when a user attempts to modify
        data with an insert, delete, or update command.  A trigger can
        instruct the system to take any number of actions when a
        specified change is attempted.  By preventing incorrect,
        unauthorized, or inconsistent changes to data, triggers help
        maintain the integrity of the database.
     
        [Sybase SQL Server Release 10.0 Transact-SQL User's Guide].
     
        (1995-02-22)