'cornerstone' definitions:

Definition of 'cornerstone'

From: WordNet
noun
The fundamental assumptions from which something is begun or developed or calculated or explained; "the whole argument rested on a basis of conjecture" [syn: basis, base, foundation, fundament, groundwork, cornerstone]
noun
A stone in the exterior of a large and important building; usually carved with a date and laid with appropriate ceremonies
noun
A stone at the outer corner of two intersecting masonry walls

Synonyms of 'cornerstone'

From: Moby Thesaurus