A triathlon at it’s simplest definition is a multisport athletic event that takes place in one day. There is a swim split, bike split and run split all with brief transition periods in between them (most often while athletes are changing gear or into run shoes from the bike split as an example).
There are a variety of different types of triathlon too. Road triathlon (most common), draft-legal road triathlon, off-road triathlon, and gravel triathlon. Within any of those races there are different distances from a supersprint (short) to a full triathlon which can be 140.6 miles.