keystone

Noun

• the central building block at the top of an arch or vault

• a central cohesive source of support and stability
  • faith is his anchor
    
  • the keystone of campaign reform was the ban on soft money
    
  • he is the linchpin of this firm
    

Related words

• Synonyms: 
  key , headstone , anchor , mainstay , backbone , linchpin , lynchpin