キーストーン

noun

keystone

Refers to the central stone at the top of an arch that locks the others in place, or metaphorically to a crucial element. Used in architecture, engineering, and figurative contexts.

• アーチのキーストーンがなければ全ぜん体たいが崩くずれてしまう。

  Without the keystone of the arch, the whole thing would collapse.

• この政せい策さくは経けい済ざい改かい革かくのキーストーンだ。

  This policy is the keystone of the economic reform.