noun - Swim-off is a head-to-head race between 2 athletes in order to break a tie conducted in a preliminary heat. The winner earns the right to compete in the finals.
In an unusual situation in Setubal Bay, the 2012 FINA Olympic 10km Marathon Swimming Qualifier will essentially be a swim-off between the 2 athletes from each country. The winner will earn a spot in the Olympic 10km Marathon Swim in London.
 USA Swimming Swim-off Definition
Swim-Offs — A swim-off is considered to be part of the total preliminary process of qualifying for the finals. In no case may a swimmer with a faster time displace another who placed ahead of him within a heat as the result of a decision by the Referee or Place Judges in accordance with Section 102.24. If this situation results in disputed qualifications, all swimmers having times tied or within the disputed times shall swim-off to qualify for the disputed place or places in the final. The swim-off shall be timed and judged in the same manner as the original event unless a subsequent timing system malfunction requires that an adjustment be made. For the seeding of finals, the times used for the swimmers involved shall be the times achieved in their original preliminary heats. This elimination may be held at any time set by the Referee, but not more than 45 minutes after the last heat of any event in which any one of these swimmers is competing in that session. In the case of a disqualification in a swim-off the swimmer so disqualified is relegated to the lowest qualifying position for which he/she is competing. Disqualification in a swim-off for a qualifying position shall not eliminate a swimmer from eligibility to compete in the accompanying finals, or as an alternate. If 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.