definition of
cornerstone

[1] #1 cornerstone n. [basis, base, foundation, fundament, groundwork] the fundamental assumptions from which something is begun or developed or calculated or explained; "the whole argument rested on a basis of conjecture". wn

[2] #2 cornerstone n. a stone in the exterior of a large and important building; usually carved with a date and laid with appropriate ceremonies. wn

[3] #3 cornerstone n. a stone at the outer corner of two intersecting masonry walls. wn