As used in this title:

The term “department” means one of the executive departments enumerated in section 1 of Title 5, unless the context shows that such term was intended to describe the executive, legislative, or judicial branches of the government.

The term “agency” includes any department, independent establishment, commission, administration, authority, board or bureau of the United States or any corporation in which the United States has a proprietary interest, unless the context shows that such term was intended to be used in a more limited sense.