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.
The term program is often used today, in conjunction with, and in part as a synonym of, an 'architectural brief'. 'Program' is more likely to refer to the broader objectives whereas 'brief' is more likely to refer to the more specific instructions. 'Program' is also used more specifically to relate to those aspects of a brief which deal with the human movement and use of a building.