Precedent
Etymology
From Latin praecedere, "to go before" (prae, "before," + cedere, "to go"). The literal sense is simply something that came earlier. English law hardened it into a term of art: an earlier ruling that binds or guides later ones, the backbone of the common-law principle of stare decisis, "to stand by things decided."
noun · /PRESS-ih-dent/
A past decision that serves as a rule or example for deciding similar cases later.
What it names
The reason a lawyer will cite a decades-old ruling in court — courts are expected to follow how comparable questions were settled before.
Example
The judge ruled reluctantly, noting that a Supreme Court precedent left her little room to decide otherwise.
Related idea
A statute is law written down and enacted in advance; a precedent is law built up case by case through decisions.