An Architectural brief in its broadest sense, is a problem a client may have, which an
architect attempts to solve. In design, a written brief is often formulated with the architect, wherein problems are appraised, constraints highlighted and requirements for further research are identified. In the early stages of the design process the brief may be continuously reappraised as the requirements become clearer - this is known as 'firming up the brief'. Ultimately, a clear set of instructions setting out the key goals and requirements will emerge which form the start point of the architect's designs and will be continually tested against, as designs are progressed. In the later stages of writing a brief, a schedule of accommodation is sometimes drawn up, that will specify precisely the number and size of rooms that will be required.
See more at Wikipedia.org...