In Newtonian reflectors, this is a mirror, and in a refractor, this is an objective lens.
The telescope’s main light-collecting element is known as the objective.  In Newtonian reflectors, this is a mirror, and in a refractor, this is an objective lens.  A larger objective means more detail and the ability to reveal dimmer targets.  The reflector design allows for a larger telescope objective at an affordable price, while a refractor of the same size would increase the price dramatically.