trigger

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The lever pressed by the finger to discharge a firearm.
2. n. A similar device used to release or activate a mechanism.
3. n. An event that precipitates other events.
4. n. Electronics A pulse or circuit that initiates the action of another component.
5. v. To set off; initiate: remarks that triggered bitter debates.
6. v. To fire or explode (a weapon or an explosive charge).

Century Dictionary and Cyclopedia

1. n. 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.
2. n. A catch to hold the wheel of a carriage on a declivity.
3. n. In ship-building, a wooden piece employed to hold up a dogshore. It is removed just before launching, when the dogshore is knocked away.

Wiktionary

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

GNU Webster's 1913

1. n. A catch to hold the wheel of a carriage on a declivity.
2. n. (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.

WordNet 3.0

1. n. a device that activates or releases or causes something to happen
2. n. an act that sets in motion some course of events
3. n. lever that activates the firing mechanism of a gun
4. v. put in motion or move to act
5. v. release or pull the trigger on