cor·ner·stone also corner stone 
(kôrnər-stōn′)
(kôrnər-stōn′)n.
1.
a. A stone at the corner of a building uniting two intersecting walls; a quoin.
b. Such a stone when inscribed with the date of the start of the construction of the building.
2. An indispensable and fundamental basis: the cornerstone of an argument.