As the term is used in the United States today, an advisory opinion is a formal opinion by a judge or judges of a supreme court, or by a supreme court, in answer to a question of law, submitted by a legislative body or a governor, a council, or a governor and council, of a state, which question is not related to nor concerned with a case or controversy in actual litigation at the time, and which does not involve private rights.
George N. Stevens,
Advisory Opinions—Present Status and an Evaluation,
34 Wash. L. Rev. & St. B.J.
Available at: https://digitalcommons.law.uw.edu/wlr/vol34/iss1/1