keystone in WordNet English dictionary

1. 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"
   anchor  mainstay  keystone  backbone  linchpin  lynchpin 
   
2. the central building block at the top of an arch or vault
   keystone  key  headstone