1. The central building block at the top of an arch or vault
   "The mason carefully positioned the keystone to complete the arch";
   - key, headstone
    
2. A central cohesive source of support and stability
   "the keystone of campaign reform was the ban on soft money";
   - anchor, mainstay, backbone, linchpin, lynchpin

Derived forms: keystones

Type of: building block, support

Part of: arch

Encyclopedia: Keystone, SD