coping stone; capstone
A stone placed on top of a wall, pillar, or roof to protect it from weathering and provide a finished appearance. Used in traditional Japanese architecture and stonework.
A coping stone is placed on top of the stone wall to prevent rainwater from seeping in.
Compound of 笠 (kasa, 'conical hat' or 'cap') and 石 (ishi, 'stone'), referring to a stone that caps or covers the top of a structure like a hat.