A keystone is the wedge-shaped stone placed at the very top of an arch that locks all the other stones together. Ancient Roman architects discovered that by placing this final stone at the arch's peak, they could make the entire structure stable and strong. Without the keystone, the arch would collapse. With it, the arch could support enormous weight, including bridges, aqueducts, and cathedral ceilings that still stand today.

This meaning led to another important use of the word: a keystone can be anything that holds a system together or makes it work. In biology, a keystone species is an animal or plant that many other species depend on. Sea otters are keystone species along the Pacific coast because they eat sea urchins, which would otherwise destroy the kelp forests where countless other creatures live. Remove the otters, and the entire ecosystem collapses.

In everyday life, you might call cooperation the keystone of a successful team, or describe a particularly important person as the keystone of an organization. Pennsylvania is called the Keystone State because of its central location among the original thirteen colonies.