keystone
//ˈkiːˌstoʊn//
Noun
keystoneSingular
keystonesPlural
1
The central wedge-shaped stone at the top of an arch that locks all the other stones in place.
The keystone at the top of the arch holds all the other stones together.
2
The most important part of a plan, idea, or system on which everything else depends.
Trust is the keystone of any successful relationship.