Cornerstone

Noun

1. A stone forming the base at the corner of a building.
2. Such a stone used ceremonially, often inscribed with the architect's and owner's names, dates and other details.

   "The cornerstone on the Flatiron Building is set on the Fifth Avenue facade."

3. That which is prominent, fundamental, noteworthy, or central.

   "Exceptional service is the cornerstone of the hospitality industry."