A special race in which the winner gets to compete in the next level of trials (typically the finals), where the competitors have not qualified for the next level by the usual elimination races.
1973, Aquatics Guide, with Official Rules and Swimming and Diving Standards, If the disqualifications leave a vacancy for the full complement of finalists, swim-offs shall be continued among the disqualified swimmers until a full complement of finalists is assured.
2008, Richard J Foster, Mark Spitz: The Extraordinary Life of an Olympic Champion , The swim-off would be open to any athlete who hadn't competed in the official 200 free. Since the swim-off would be the last event at the Trials, the upshot allowed Mark to enter it without affecting any of his other races.
2012, Dan Stephenson, The Underwater Window, They came up with a compromise where he could compete in a special swim-off two months later.
