A keystone is the central, wedge-shaped stone at the apex of an arch. It is the final piece placed during construction, locking all the other stones into position and enabling the arch to bear weight. In a broader, metaphorical sense, a “keystone” refers to something essential to the structure, function, or integrity of a system or organization. In architectural construction, the keystone plays a critical role in maintaining the stability of the entire structure; without it, the arch would collapse.