What is Ratio Decidendi?

Legal Definition
Ratio decidendi (Latin plural rationes decidendi) is a Latin phrase meaning "the reason" or "the rationale for the decision". The ratio decidendi is "the point in a case that determines the judgment" or "the principle that the case establishes".

In other words, ratio decidendi is a legal rule derived from, and consistent with, those parts of legal reasoning within a judgment on which the outcome of the case depends.

It is a legal phrase which refers to the legal, moral, political and social principles used by a court to compose the rationale of a particular judgment. Unlike obiter dicta, the ratio decidendi is, as a general rule, binding on courts of lower and later jurisdiction—through the doctrine of stare decisis. Certain courts are able to overrule decisions of a court of coordinate jurisdiction—however, out of interests of judicial comity, they generally try to follow coordinate rationes.

The process of determining the ratio decidendi is a correctly thought analysis of what the court actually decided—essentially, based on the legal points about which the parties in the case actually fought. All other statements about the law in the text of a court opinion—all pronouncements that do not form a part of the court's rulings on the issues actually decided in that particular case (whether they are correct statements of law or not)—are obiter dicta, and are not rules for which that particular case stands.
-- Wikipedia
Legal Definition
Latin, "rationale for the decision." The term refers to a key factual point or chain of reasoning in a case that drives the final judgment. When considering earlier cases as precedent, courts often ask parties to be very clear about how they interpret the main guiding principle or ratio decidendi of the earlier case.
Legal Definition
The ground of decision. The point in a case which determines the judgment.
-- Black's Law Dictionary
Legal Definition
The ground of the decision.
-- Ballentine's Law Dictionary