What is the meaning of cornerstone?

noun

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