Trigger
The specific thing that starts the behaviour — a bike, a doorbell, a man in a helmet, you picking up your keys.
Also called: triggers · what sets my dog off
Owners usually describe the reaction and not the trigger: "he barks at everything". He almost never barks at everything. Watch for a week and it turns out to be delivery riders, or dogs that stare, or the lift doors opening, or being approached head-on when he is on a lead and cannot move away.
Naming the trigger precisely is most of the work, because a plan for "barks at everything" is not a plan. A plan for "barks at men carrying bags between 6 and 9pm in the corridor" is one you can actually rehearse.
Triggers are also stackable and they are not always visible: heat, a missed walk, a house full of guests and a sore hip all lower the bar before the delivery rider ever arrives.