A creative brief is a short, structured document that defines what a creative project must achieve, for whom, and within which constraints — before any work begins.
A creative brief is the contract of understanding between the people who need the work and the people who make it. It typically covers background, objective, target audience, single-minded proposition, tone of voice, deliverables, mandatories, budget and timing.
A good creative brief is short. If it takes more than two pages, the thinking behind it is usually not finished yet. The discipline of the brief is not in writing everything down — it is in deciding what matters most.
The brief is also the reference point for evaluation. When feedback rounds start drifting, the brief is what you return to: does this work solve the problem we agreed to solve?