doctrinal (adj.)

late 14c., "the body of principles, dogmas, etc., in a religion or field of knowledge," from Old French doctrine (12c.) "teaching, doctrine" and directly from Latin doctrina "a teaching, body of teachings, learning," from doctor "teacher" (see doctor (n.)) + -ina, fem. of -inus, suffix forming fem. abstract nouns (see -ine (1)).

The notion is "whatever is taught or laid down as true by a master or instructor," hence "any set of principles held as true." In Middle English, it could be used generally for "learning, instruction, education." In U.S. history, the Monroe doctrine was put forward in a message to Congress Dec. 2, 1823; the exact phrase is attested by 1848.