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

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "cornerstone":
   bench mark, cardinal point, chief thing, climax, core, crisis,
   critical point, crux, essence, essential, essential matter,
   first stone, footstone, foundation stone, fundamental, gist,
   gravamen, great point, headstone, heart, high point,
   important thing, issue, kernel, keystone, landmark, main point,
   main thing, material point, meat, milestone, nub, pith, pivot,
   quoin, real issue, salient point, sine qua non, substance,
   substantive point, the bottom line, the point, turning point