definition of
keystone

[1] #1 keystone n. [anchor, mainstay, backbone, linchpin, lynchpin] 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". wn

[2] #2 keystone n. [key, headstone] the central building block at the top of an arch or vault. wn