Once the highest court in state A adopts a definition from the Restatement of the Law, this definition serves as what type of authority?

When a state's highest court adopts a definition from the Restatement of the Law, that definition becomes mandatory authority for all lower courts within the same jurisdiction. Mandatory authority is a source of law that must be followed by lower courts in the same jurisdiction, as it represents a binding interpretation of the law that has been established by the highest court.

The Restatement of the Law is a collection of legal principles that is considered highly influential and provides guidance on various legal issues. When the highest court incorporates a definition from the Restatement, it elevates that definition to the status of law within that state, thus requiring all lower courts to adhere to this interpretation in future cases. This is fundamentally distinct from persuasive authority, which may be influential but does not bind courts to follow it, and secondary authority, which includes resources like legal textbooks and treatises that provide commentary on the law but do not have legal force. Therefore, when a definition from the Restatement is adopted by the highest court, it is recognized as mandatory authority that must be followed by subordinate courts in that state.