cornerstone
//ˈkɔːnəˌstəʊn//
Noun
cornerstoneSingular
cornerstonesPlural
1
A stone at the corner of a building, especially one laid ceremonially to mark the beginning of construction.
The mayor laid the cornerstone of the new library.
2
An essential or fundamental element; the basis or foundation upon which something is built or developed.
Trust is the cornerstone of any successful relationship.