Definition of Cornerstone

Noun: cornerstone  'kor-nu(r),stown

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

See also:
corners
cornerstones