A triathlon is a multi-sport event involving the completion of three continuous and sequential endurance events. While many variations of the sport exist, triathlon, in its most popular form, involves swimming, cycling, and running in immediate succession over various distances. Triathletes compete for fastest overall course completion time, including timed "transitions" between the individual swim, bike, and run components. The word "triathlon" is of Greek origin from τρεις or trei (three) and αθλος or athlos (contest).

Triathlon races vary in distance. According to the International Triathlon Union, and USA Triathlon, the main international race distances are Sprint distance (750 metres (0.47 mi) swim, 20 kilometres (12 mi) bike, 5 kilometres (3.1 mi) run), Intermediate (or Standard) distance, commonly referred to as "Olympic distance" (1.5 kilometres (0.93 mi) swim, 40 kilometres (25 mi) ride, 10 kilometres (6.2 mi) run), the Long Course (1.9 kilometres (1.2 mi) swim, 90 kilometres (56 mi) ride, 21.1 kilometres (13.1 mi) run, such as the Half Ironman), and Ultra Distance (3.8 kilometres (2.4 mi) swim, 180 kilometres (110 mi) ride, and a marathon: 42.2 kilometres (26.2 mi) run); the most recognized branded Ultra Distance is the Ironman triathlon.