It is an Olympic sport that gathers swimming, cycling and running, without a rest between each and in that order. The switch between disciplines is called a transition. The transition from swimming to cycling is known as T1 and the one from cycling to marathon as T2.
The sport was invented during the late 70s, after a bet was made between members of the US Navy. It first became an Olympic sport at the 2000 Sidney Olympic Games, with 50 men and 50 women competing.