One stop of light is usually defined as halving or doubling the amount of light entering the camera, depending on the shutter speed, aperture, or ISO changes made.
What no one dares stop calling the Special Relationship has been defined down the years by images - Ronald Reagan and Margaret Thatcher dancing, George W Bush joking about sharing Colgate with a grinning Tony Blair squeezed into crotch hugging chinos and years later spinning a be-suited and grim-faced Gordon Brown around in a golf buggy.