Definition of thirty-year rule in English:
A rule that public records may be open to inspection after a lapse of thirty years.
- The role played by the British government during this critical period of the Cold War was widely ignored until the opening of the British archives for the late 1950s and early 1960s, under the thirty-year rule.
- Files recently released to the National Archives under the thirty-year rule confirm an increased emphasis by MI5's F Branch on industrial and trade union matters.
- The annual release of cabinet papers under the thirty-year rule has been pushed off the front pages by the disaster.