WordNet (r) 3.0 (2006):

cornerstone
    n 1: 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]
    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