noun the fundamental assumptions from which something is begun or developed or calculated or explained.
— synonyms: basis, base, foundation, fundament, groundwork
noun a stone in the exterior of a large and important building.
noun a stone at the outer corner of two intersecting masonry walls.
Examples
“The cornerstone on the Flatiron Building is set on the Fifth Avenue facade.”
“Exceptional service is the cornerstone of the hospitality industry.”