keystone

/ˈkiːˌstoʊn/
noun
  1. The central stone at the top of an arch that holds the other stones in place.
    • Without the keystone, the arch would collapse.
    • The mason carefully set the keystone into the bridge.
    • The ancient Romans carved the keystone with a decorative emblem.
  2. The most important part of a system, plan, or idea that supports everything else.
    • Education is the keystone of a successful society.
    • The new policy became the keystone of the company's growth strategy.
    • Trust is the keystone of any strong relationship.