Brief
A concise document describing a project's goals, requirements, constraints, and expectations.
In practice
A brief is the short document that states what a project must achieve: its goal, audience, deliverables, constraints such as budget and deadline, and how success will be judged. It is agreed before design starts and referred back to throughout. A brief for a festival poster might specify the audience, the required information, the print size, mandatory sponsor logos and the date files are due.
Often confused with Brandbook
A brief describes one project's goals and constraints; a brandbook gives lasting rules for applying a brand's identity across all projects.
Try this
Write a five-line brief for redesigning your phone's home screen: goal, user, constraints, deliverable and deadline.