Executive privilege is "a concept invoked by members of the executive branch of the government to justify withholding evidence and other communicative materials from the legislative and judicial branches." Since the presidency of George Washington, the executive has attempted to withhold information from the other two branches.
Jean M. D'Ovidio,
Executive Privilege: Historic Scope and Use in the Watergate and Environmental Protection Agency Hearings,
U. Rich. L. Rev.
Available at: http://scholarship.richmond.edu/lawreview/vol18/iss1/10