trigger

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The lever pressed by the finger to discharge a firearm.
• noun A similar device used to release or activate a mechanism.
• noun An event that precipitates other events.
• noun Electronics A pulse or circuit that initiates the action of another component.
• transitive verb To set off; initiate.
• transitive verb To fire or explode (a weapon or an explosive charge).

from The Century Dictionary.

• noun Any device by means of which a catch or spring is released and a trap sprung or other mechanism set in action; specifically, in firearms, a small projecting tongue of steel which, when pressed, liberates the hammer of the lock; by extension, in crossbows and similar arms, the lever which, when pressed, liberates the string of the bow. See hair-trigger, and cuts under gun, revolver, and rifle.
• noun A catch to hold the wheel of a carriage on a declivity.
• noun In ship-building, a wooden piece employed to hold up a dogshore. It is removed just before launching, when the dogshore is knocked away.

from the GNU version of the Collaborative International Dictionary of English.

• noun A catch to hold the wheel of a carriage on a declivity.
• noun (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.
• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A finger-operated lever used to fire a gun.
• noun A similar device used to activate any mechanism.
• noun An event that initiates others, or incites a response.
• noun psychology An event, experience or other stimulus that initiates a traumatic memory or action in a person.
• noun electronics A pulse in an electronic circuit that initiates some component.
• noun computing An SQL procedure that may be initiated when a record is inserted, updated or deleted; typically used to maintain referential integrity.
• noun online gaming A text string that, when received by a player, will cause the player to execute a certain command.
• noun archaic A catch to hold the wheel of a carriage on a declivity.
• verb transitive to fire a weapon
• verb transitive to initiate something

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a device that activates or releases or causes something to happen
• noun an act that sets in motion some course of events
• noun lever that activates the firing mechanism of a gun
• verb put in motion or move to act
• verb release or pull the trigger on