keystone
nounUK /ˈkiːstəʊn/
US /ˈkiːstoʊn/
The keystone is the special middle stone at the top of an arch that holds all the other stones in place.
The workers carefully set the keystone at the top of the stone bridge.
OTHER MEANINGS
2. The most important part of a plan, system, or idea that everything else depends on.
Trust is the keystone of a good friendship.
3. A central stone at the top of an arch that locks the other stones together.
Without the keystone, the arch would collapse.
4. A species that greatly affects the other plants and animals in its habitat.
Sea otters are a keystone species in kelp forests.