cornerstone

noun

1. A stone forming the base at the corner of a building.

   “Clutch it like a cornerstone Otherwise it all comes down Justify denials and Grip 'em to the lonesome end”

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. (figuratively)That which is prominent, fundamental, noteworthy, or central.

   “Exceptional service is the cornerstone of the hospitality industry.”

   “That is the cornerstone of any meaningful debate about budgets and projects, regulations and policies.”

   “Jewish immigrants came together with other foreign-born groups in organizations like the Industrial Workers of the World, helping to create one of the cornerstones of the American left.”