A worl record is the best performance in a certain discipline, usually a sports event. In the United States the form world's record was formerly more common. The term world best was also briefly in use. The latter term is still used in athletics (track and field) to describe performances not recognized as an official world record: either because the event is a non-qualifying event (e.g. the 500 yards run), or because it does not fulfill other criteria of an otherwise qualifying event (e.g. the Great North Run half-marathon, which has an excessive downhill gradient).