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key + stone
A keystone.
- (countable) A keystone is the most important part of something, on which everything else depends.
- Trust is the keystone of any strong relationship.
- (usually before a noun) A keystone part is the most important or foundational part of a system or plan.
- Education is the keystone policy of the new government.
- Keystone species play a critical role in maintaining the structure of an ecosystem.
- (countable) A keystone is the central stone at the top of an arch that holds the other stones in place.
- The old bridge's arch was held up by a single keystone.
