keystone

Noun

Synonyms: anchor mainstay backbone linchpin lynchpin key headstone

WordNet 3.0

Noun (2)

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”Synonyms: anchor mainstay backbone linchpin lynchpin
the central building block at the top of an arch or vaultSynonyms: key headstone

Webster's Dictionary (GCIDE)

Keystone \Key"stone`\, n. (Arch.) The central or topmost stone of an arch. This in some styles is made different in size from the other voussoirs, or projects, or is decorated with carving. See Illust. of Arch. [1913 Webster]