noun (n)

• a central cohesive source of support and stability(noun.cognition)

  Example:
  The keystone of campaign reform was the ban on soft money.
  

  Synonym:
  anchor, backbone, linchpin, lynchpin, mainstay
  

  source: wordnet30
• the central building block at the top of an arch or vault(noun.artifact)

  Synonym:
  headstone, key
  

  source: wordnet30
• The central or topmost stone of an arch. This in some styles is made different in size from the other voussoirs, or projects, or is decorated with carving. See Illust. of Arch.(noun)

  source: webster1913