What is Bureau?

Legal Definition
A French word, which literally means a large writing table. It is used figuratively for the place where business is transacted: it has been borrowed by us, and used in nearly the same sense; as, the bureau of the secretary of state. Vide Merl. Repert. h. t.
-- Bouviers Law Dictionary
Legal Definition
An office for the transaction of business. A name given to the several departments of the executive or administrative branch of government or to their larger subdivisions. In re Strawbridge, 39 Ala. 375.
-- Black's Law Dictionary
Legal Definition
A business office; a government department.
-- Ballentine's Law Dictionary