The area where you switch from swim to bike (T1) and from bike to run (T2). Performance analyses count both transitions as race components in their own right.
Transition is called the fourth discipline because it is one: performance analyses of Olympic-distance racing break a race into five components — three legs plus T1 and T2 — and derive target splits from them. How much it weighs depends on the format: over sprint distance the elite complete T1 and T2 together in under two minutes, while everyday athletes often lose three to six — over a race under two hours that share is substantial. The mistake is nearly always the same: never rehearsed, then improvised on adrenaline. The fix costs nothing — a fixed spot for helmet, shoes and nutrition, the same order every time, elastic laces, the bike loaded at check-in. Two things that get missed: the helmet goes on first and comes off last. And at long distance five to ten minutes per transition is normal — there the zone is a reset point, not a race against the clock.