Separating trigger from setup is more useful than it sounds. The setup says this is an area worth trading; the trigger says now, at this price, with this stop. One can be right while the other is wrong.
Common triggers include a break of the previous bar's extreme, a close through a level, a reclaim after a sweep, or a lower-timeframe break-of-structure. Each gives a different entry price and therefore a different risk-reward-ratio from the same idea.
Having a defined trigger also prevents the two most expensive habits: entering early because the setup looks obvious, and entering late after watching price run. Both are failures of timing rather than analysis, and both are fixable by writing the trigger down.
Related: setup, confirmation-signal, trade-location, break-of-structure, invalidation-level