Ratio decidendi, often shortened to ratio, is Latin for the “the reason for deciding”. In current Legal English usage it means the principle, ground or reason for the decision in a case, often a statement of law. Put another way, it is the rationale (=an explanation of the fundamental reasons or basis for something) of the decision. In the common law system of precedents then, the ratio decidendi not only binds the parties in the matter decided by the court, but also serves as principle with the force of law that will be applied to later cases.
It is not always easy to identify the ratio decidendi of a case, and not every statement in the judgment is binding as ratio decidendi. What is more, the ratio decidendi of a decision may be narrowed or widened by a judge. For example, a modern court may note that under the ratio decidendi of the case in which it adopted a certain rule, only the immediate family members of the victims were entitled to claim certain types of damages.
Note that ratio decidendi is distinguishable from obiter dictum, which is something said in a judgment that is not essential to the decision and which is not binding precedent. The plural form is rationes decidendi, and is very rarely used.