A debrief is the structured review held after a project ends, capturing what worked, what did not, and what should change in the next brief.
Debriefs turn one project's pain into the next project's process. They work best when they are scheduled at kickoff rather than improvised at the end, and when they cover the briefing quality itself, not only the output.
Three questions carry most of the value: What did we assume that turned out to be wrong? Where did we lose the most time? What would we put in the brief if we started again today?