Which major Space Shuttle component was not reusable?

The Space Shuttle’s external tank was the major component that was not reusable.

The tank stored the liquid hydrogen and liquid oxygen used by the orbiter’s three RS-25 main engines. It also formed the structural backbone of the launch stack, connecting the orbiter with the two solid rocket boosters and carrying aerodynamic and launch loads.

After main engine cutoff, the external tank was jettisoned roughly 10 seconds later. It re-entered the atmosphere and was destroyed rather than recovered. This made it the Shuttle’s one major expendable element, even though the orbiter, main engines, and solid rocket boosters were designed for reuse.

The phrase “reusable spacecraft” therefore needs qualification. The Shuttle system reused its most complex and expensive hardware, but it still consumed an external tank on every launch. The solid boosters were recovered after splashing down, while the engines were removed from the orbiter and refurbished for later missions.