triathlon UK /tɹaɪˈæθlɒn/US /tɹaɪˈæθˌlɑn/ noun3 •An athletics event in which contestants compete in swimming, cycling and running in turn. •Generally, a sports event in which contestant compete in a combination of three sports. •(historical) A former Olympic athletics event in which contestants compete in long jump, shot put, and 100-yard dash. formstriathlons