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Oxford:

the system of laws and basic principles that a state, a country or an organization is governed by

Webster:

the system of beliefs and laws by which a country, state, or organization is governed 

Cambridge:

the set of laws and principles that a country's government must obey

Longman:

a set of basic laws and principles that a country or organization is governed by

Macmillan:

a set of basic laws or principles for a country that describe the rights and duties of its citizens and the way in which it is governed