keystone

Noun

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.