A trigger is a mechanism that actuates the function of a ranged weapon such as a firearm, airgun, crossbow, or speargun. The word may also be used to describe a switch that initiates the operation of other non-shooting devices such as a trap, a power tool, or a quick release. A small amount of energy applied to the trigger leads to the release of much more energy.

Most triggers use a small flat or slightly curved lever (called the trigger blade) depressed by the index finger, but some weapons such as the M2 Browning machine gun or the Iron Horse TOR ("thumb-operated receiver") use a push-button-like thumb-actuated trigger design, and others like the Springfield Armory M6 Scout use a squeeze-bar trigger similar to the "ticklers" on medieval European crossbows. Although the word "trigger" technically implies the entire mechanism (known as the trigger group), colloquially it is usually used to refer specifically to the trigger blade.

Most firearm triggers are "single-action", meaning that the trigger is designed only for the single function of disengaging the sear, which allows for a spring-tensioned hammer/striker to be released. In "double-action" firearm designs, the trigger also performs the additional function of cocking the hammer – and there are many designs where the trigger is used for a range of other functions. Furthermore, triggers can be divided into direct triggers (also called single-stage triggers) and which are popular for hunting, and pressure triggers (also called two-stage triggers which are popular on competition rifles.

Firearms use triggers to initiate the firing of a cartridge seated within the gun barrel chamber. This is accomplished by actuating a striking device through a combination of mainspring (which stores elastic energy), a trap mechanism that can hold the spring under tension, an intermediate mechanism to transmit the kinetic energy from the spring releasing, and a firing pin to eventually strike and ignite the primer. There are two primary types of striking mechanisms – hammer and striker. A hammer is a pivoting metallic component subjected to spring tension so when released will swing forward to strike a firing pin like a mallet hitting a punch/chisel, which then relays the hammer impulse by moving forward rapidly along its longitudinal axis. A striker is essentially a firing pin directly loaded to a spring, eliminating the need to be struck by a separate hammer. The firing pin/striker then collides into the cartridge primer positioned ahead of it, which contains shock-sensitive compounds (e.g., lead styphnate) that sparks to ignite the propellant powder within the cartridge case and thus discharges the projectile. The trapping interface between the trigger and the hammer/striker is typically referred to as the sear surface. Variable mechanisms will have this surface directly on the trigger and hammer or have separate sears or other connecting parts.

The trigger pull can be divided into three mechanical stages:

When considering the practical accuracy of a firearm, the trigger takeup is often considered the least critical stage of the trigger pull. Often triggers are classified as either single-stage or two-stage based on the takeup.

A single-stage trigger is often called direct trigger and is popular on hunting rifles. A two-stage trigger is often called pressure trigger and is popular on competition rifles.

Some fully adjustable triggers can be adjusted to function as either a single-stage or two-stage trigger by adjusting the takeup. Setting the takeup travel (also known as the first stage) to near zero essentially makes the trigger a single-stage trigger. Some single-stage triggers (e.g., Glock Safe Action trigger, Savage AccuTrigger) have an integral safety with a noticeable spring resistance that can functionally mimic a two-stage trigger.