In its earliest usage, trigger refers to a mechanical mechanism, the pulling or pushing of which sets a device into action. This includes the lever that sets off the chain of events that fires a gun, the release mechanisms on a crossbow, or the lever that engages an animal trap.

Today trigger is also used in a wider sense to refer to a precipitating cause of some event. The "trigger" (noun) is said to "trigger" (verb) the event.

Note that, in this wider sense, "trigger" need not be a necessary condition for the event. To the extent that it is a sufficient condition, this is only relative to some implied background conditions taken as given. (See necessary and sufficient conditions.)

Examples of triggers:

• If one adds a seed crystal to a supersaturated solution, the crystal will "trigger" the precipitation of the dissolved material.
• In the almost cliche idea of a butterfly flapping its wings and thereby causing a great weather disturbance (see chaos theory), the butterfly can be said to have "triggered" the disturbance.
• An event which causes an allergic reaction or other untoward psychological/physical event is also referred to as a trigger. For example, exposure to cat dander may trigger an asthma attack in an allergic person, and persons with post-traumatic stress disorder may experience extreme distress after being exposed to their personal trigger.
• In computer programming, software may be set up so that certain events (e.g. an insert, an update or a delete of a database) trigger the invocation of particular procedures.
• In ethology, a trigger is a cue in the environment that causes the "release" of a (genetically predetermined) behavior. An example is the pecking behavior of young Herring gulls , elicited by a red spot on the beak of a parent. In this context, the trigger is also referred to as an eliciting stimulus.

The use of the term trigger for oscilloscopes is explained in the relevant article.

Trigger was the name of Roy Rogers' horse.