a stone that forms the top of wall or building

A stone laid horizontally over a burial chamber. The large, flattened stone seen atop three or more upright stones in a dolmen.

Horizontal stone on top of chamber, passage or dolmen; dressed or otherwise

topmost stone in structure.

Coping on top of a wall.

A capstone is the top covering stone of a cist. A cist is a setting of stones, containing a human burial or cremation.

The capstone is the highest also referred to as apex, rock or mount of a structure. Often they have "doorways", the capstone would lock or wedge the rest of the rocks tightly together, giving strength, much like a wedge when driven into a tight spot. In an arch, the capstone, or sometimes called the keystone or coping stone, holds the arch together.