The ratio decidendi is the principle of law on the basis of which a case has been decided. From the Latin, “reason for the decision”.
If novel, and binding on other courts, the ratio decidendi justifies reporting the case in a law report, the headnote of which should ideally contain an accurate and authoritative statement of the ratio. (This will usually be set out in one or more paragraphs commencing with the word “Held“…)
To be distinguished from obiter dicta, which are observations of interest but not determinative of the case and not binding on other courts.