Final answer:
The first voussoir resting on the impost of an arch is called the keystone.
Step-by-step explanation:
An arch is composed of identical wedge-shaped stones, called voussoirs. The keystone is the voussoir at the top of the arch that holds everything in place. It is the first stone to be placed on the impost, which is the topmost part of the column or pier that supports the arch.