English to English

noun (n)

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

  Synonym:
  key, keystone
  

  source: wordnet30
• a stone that is used to mark a grave(noun.artifact)

  Synonym:
  gravestone, tombstone
  

  source: wordnet30
• The principal stone in a foundation; the chief or corner stone.(noun)

  source: webster1913