The central wedge-shaped stone at the crown of an arch, often enlarged or carved as an architectural ornament.
A keystone is the central wedge-shaped voussoir at the crown of an arch, or the central stone at the meeting of a vault. It completes the arch structurally and may be visually emphasized by increased size, projection, or carved decoration.
Keystones have long provided an important location for architectural sculpture. Faces, masks, coats of arms, foliage, scrolls, and other ornaments may be carved directly into the keystone or applied to its face.
Etymology
English compound: key + stone, referring to the central stone that locks the arch together.
Sculptor Notes
A carved keystone must work simultaneously as structure and image. Its wedge shape, projection, viewing height, and relationship to the surrounding voussoirs all affect the composition.
References
https://www.getty.edu/vow/AATFullDisplay?subjectid=300001183