In an agent or other software system, an event can trigger behaviour such as a method being executed or a message being sent. The term trigger is sometimes used as a noun to refer to the condition that gives rise to the action (e.g. "the trigger for turning on the heating is when the temperature is below 5 degrees"") and sometimes as a verb for the invoking of the action (e.g. "the temperature drop triggered the hearting system to start").