What word best matches the definition below The rules and principles by which a nation is governed?

The word that best matches the definition "The rules and principles by which a nation is governed" is Constitution.

Here's why:

* Constitution: This is the fundamental law of a nation, outlining the basic structure of the government, its powers, and the rights of the people.

While other words might partially relate, "Constitution" is the most direct and comprehensive term for the governing principles of a nation.