Ratio decidendi
The legal principle or reasoning that is essential to a court's decision and is binding on lower courts.
Ratio decidendi (Latin for 'the reason for deciding') is the part of a judgment that establishes a binding legal rule. It is the principle the court had to decide in order to reach its result.
Identifying the ratio is the core skill of case-law research: it is what a later court must follow, as opposed to obiter dicta, which are persuasive but not binding.