What is Advisory Opinion?

Legal Definition
An advisory opinion is an opinion issued by a court or a commission like an election commission that does not have the effect of adjudicating a specific legal case, but merely advises on the constitutionality or interpretation of a law. Some countries have procedures by which the executive or legislative branches may certify important questions to the judiciary and obtain an advisory opinion. In other countries or specific jurisdictions, courts may be prohibited from issuing advisory opinions.
-- Wikipedia
Legal Definition
A court's nonbinding interpretation of law. It states the opinion of a court upon a legal question submitted by a legislature, government official, or another court. Federal courts cannot issue advisory opinions because of the Constitution's case-or-controversy requirement.

See, for example, the advisory opinion of the Massachusetts Supreme Judicial Court regarding civil unions.
Related terms:
Legal Definition
The ways a court can rule in a case. It relies on the evidence so it is not binding until the arguments are finished.
Legal Definition
An opinion of a higher court upon a point before a lower one.
-- Ballentine's Law Dictionary